Semiconductor device and method for manufacturing the same

ABSTRACT

A highly reliable semiconductor device and a method for manufacturing the semiconductor device are provided. In a semiconductor device including a bottom-gate transistor in which an insulating layer functioning as a channel protective film is provided over an oxide semiconductor film, elements contained in an etching gas can be prevented from remaining as impurities on a surface of the oxide semiconductor film by performing impurity-removing process after formation of an insulating layer provided over and in contact with the oxide semiconductor film and/or formation of source and drain electrode layers. The impurity concentration in the surface of the oxide semiconductor film is lower than or equal to 5×10 18  atoms/cm 3 , preferably lower than or equal to 1×10 18  atoms/cm 3 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

Note that the semiconductor device in this specification refers to alldevices that can function by utilizing semiconductor characteristics,and electro-optic devices, semiconductor circuits, and electronicappliances are all semiconductor devices.

2. Description of the Related Art

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. The transistors are applied to a wide range ofsemiconductor devices such as an integrated circuit (IC) and an imagedisplay device (also simply referred to as display device). Asilicon-based semiconductor material is widely known as a material for asemiconductor thin film applicable to a transistor. As another material,an oxide semiconductor has been attracting attention.

For example, a transistor including a semiconductor layer formed of anamorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) (anIGZO-based amorphous oxide) is disclosed (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2011-181801

SUMMARY OF THE INVENTION

Improvement in reliability is important for commercialization ofsemiconductor devices including transistors formed using an oxidesemiconductor.

However, a semiconductor device includes a plurality of thin filmscomplicatedly stacked, and is manufactured using a variety of materials,methods, and steps. Therefore, an employed manufacturing process maycause shape defects or degradation of electric characteristics of asemiconductor device which is to be provided.

In view of the above problem, an object of one embodiment of the presentinvention is to provide a highly reliable semiconductor device whichincludes a transistor formed using an oxide semiconductor.

In a semiconductor device including a bottom-gate transistor in which aninsulating layer functioning as a channel protective film is providedover an oxide semiconductor film, elements (e.g., chlorine and boron)contained in an etching gas which is used for forming the insulatinglayer provided over and in contact with the oxide semiconductor filmand/or source and drain electrode layers are/is prevented from remainingas impurities on a surface of the oxide semiconductor film.Specifically, the following embodiment can be employed, for example.

One embodiment of the present invention is a method for manufacturing asemiconductor device, including the steps of: forming a gate electrodelayer over an insulating surface; forming a gate insulating film overthe gate electrode layer; forming an island-shaped oxide semiconductorfilm over the gate insulating film; forming an insulating layeroverlapping with the gate electrode layer and in contact with theisland-shaped oxide semiconductor film; forming a conductive filmcovering the island-shaped oxide semiconductor film and the insulatinglayer; processing the conductive film by plasma treatment using anetching gas containing a halogen element to form a source electrodelayer and a drain electrode layer, so that part of the oxidesemiconductor film is exposed; and performing impurity-removingtreatment on the exposed oxide semiconductor film to remove the elementcontained in the etching gas.

One embodiment of the present invention is a method for manufacturing asemiconductor device, including the steps of: forming a gate electrodelayer over an insulating surface; forming a gate insulating film overthe gate electrode layer; forming an island-shaped oxide semiconductorfilm over the gate insulating film; forming an insulating layer coveringthe island-shaped oxide semiconductor film; processing the insulatinglayer by plasma treatment using an etching gas containing a halogenelement to form an insulating layer functioning as a channel protectivefilm in a position overlapping with the gate electrode layer; performingimpurity-removing treatment on the oxide semiconductor film to removethe element contained in the etching gas; forming a conductive filmcovering the island-shaped oxide semiconductor film and the insulatinglayer functioning as the channel protective film; and processing theconductive film to form a source electrode layer and a drain electrodelayer covering edge portions of the oxide semiconductor film in achannel width direction.

In the method for manufacturing a semiconductor device, a chlorineconcentration in a surface of the oxide semiconductor film subjected tothe impurity-removing treatment is preferably lower than or equal to5×10¹⁸ atoms/cm³.

In the method for manufacturing a semiconductor device, oxygen plasmatreatment, dinitrogen monoxide plasma treatment, or cleaning treatmentwith a diluted hydrofluoric acid solution can be performed as theimpurity-removing treatment.

One embodiment of the present invention is a method for manufacturing asemiconductor device, including the steps of: forming a gate electrodelayer over an insulating surface; forming a gate insulating film overthe gate electrode layer; forming an island-shaped oxide semiconductorfilm over the gate insulating film; forming an insulating layer coveringthe island-shaped oxide semiconductor film; processing the insulatinglayer by plasma treatment using an etching gas containing a halogenelement to form an insulating layer functioning as a channel protectivefilm in a position overlapping with the gate electrode layer; performingfirst impurity-removing treatment on the oxide semiconductor film toremove the element contained in the etching gas; forming a conductivefilm covering the island-shaped oxide semiconductor film and theinsulating layer functioning as the channel protective film; processingthe conductive film by plasma treatment using an etching gas containinga halogen element to form a source electrode layer and a drain electrodelayer, so that part of the oxide semiconductor film is exposed; andperforming second impurity-removing treatment on the exposed oxidesemiconductor film to remove the element contained in the etching gas.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer provided over an insulating surface, agate insulating film provided over the gate electrode layer, anisland-shaped oxide semiconductor film provided over the gate insulatingfilm, an insulating layer provided over the oxide semiconductor film andoverlapping with the gate electrode layer, and a source electrode layerand a drain electrode layer in contact with the oxide semiconductor filmand the insulating layer. In the semiconductor device, a length of eachof the source electrode layer and the drain electrode layer in thechannel width direction is smaller than a length of the oxidesemiconductor film in the channel width direction, and a chlorineconcentration in a surface of the oxide semiconductor film is lower thanor equal to 5×10¹⁸ atoms/cm³.

In the semiconductor device, a region of the oxide semiconductor filmwhich overlaps with the insulating layer, the source electrode layer, orthe drain electrode layer may have a larger thickness than a region ofthe oxide semiconductor film which does not overlap with any of theinsulating layer, the source electrode layer, or the drain electrodelayer.

Further, in the semiconductor device, any region of the oxidesemiconductor film overlaps with at least one of the insulating layer,the source electrode layer, and the drain electrode layer.

One embodiment of the present invention is a semiconductor deviceincluding a gate electrode layer provided over an insulating surface, agate insulating film provided over the gate electrode layer, anisland-shaped oxide semiconductor film provided over the gate insulatingfilm, an insulating layer provided over the oxide semiconductor film andoverlapping with the gate electrode layer, and a source electrode layerand a drain electrode layer in contact with the oxide semiconductor filmand the insulating layer. In the semiconductor device, the sourceelectrode layer and the drain electrode layer cover edge portions of theoxide semiconductor film in a channel width direction, and a chlorineconcentration in a surface of the oxide semiconductor film is lower thanor equal to 5×10¹⁸ atoms/cm³.

In the semiconductor device, a region of the oxide semiconductor filmwhich overlaps with the insulating layer may have a larger thicknessthan a region of the oxide semiconductor film which overlaps with thesource electrode layer or the drain electrode layer.

Plasma treatment using an etching gas containing a halogen element isfavorably employed for pattern formation of a film which is over and incontact with an oxide semiconductor film, such as an insulating layerfunctioning as a channel formation region, a source electrode layer, ora drain electrode layer. However, if the oxide semiconductor film isexposed to the etching gas containing a halogen element, oxygen in theoxide semiconductor film is extracted by the halogen element (e.g.,chlorine or fluorine) contained in the etching gas in some cases, whichmight cause oxygen vacancies to be formed in the vicinity of aninterface of the oxide semiconductor film. Such oxygen vacancies in theoxide semiconductor film might cause a backchannel of the oxidesemiconductor film to have lower resistance (n-type conductivity),resulting in formation of a parasitic channel.

For example, in the case where an oxide semiconductor materialcontaining indium is used for the oxide semiconductor film and anetching gas containing boron trichloride (BCl₃) is used for processingthe source electrode layer and the drain electrode layer which areprovided in contact with the oxide semiconductor film, an In—O—In bondin the oxide semiconductor film and Cl contained in the etching gassometimes react with each other, so that a film including an In—Cl bondand an In element from which oxygen is detached may be formed. Since theIn element from which oxygen is detached has a dangling bond, an oxygenvacancy exists in the portion of the oxide semiconductor film, fromwhich oxygen is detached.

Further, in the case where the etching gas containing a halogen elementalso contains an element (e.g., boron) that is not halogen, the elementthat is not halogen can cause the backchannel of the oxide semiconductorfilm to have lower resistance (n-type conductivity).

According to one embodiment of the present invention, after theinsulating layer and/or the source electrode layer and the drainelectrode layer which are provided over the oxide semiconductor film areformed by etching processing, impurity-removing treatment is performed;thus, elements (e.g., chlorine and boron) that are contained in anetching gas and can cause the oxide semiconductor film to have lowerresistance are removed. Accordingly, the reliability of thesemiconductor device can be improved.

A highly reliable semiconductor device which includes a transistorincluding an oxide semiconductor is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a plan view and cross-sectional views illustratingone embodiment of a semiconductor device.

FIGS. 2A to 2E are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device.

FIGS. 3A to 3C are a plan view and cross-sectional views illustratingone embodiment of a semiconductor device.

FIGS. 4A to 4C are a plan view and cross-sectional views illustratingone embodiment of a semiconductor device.

FIGS. 5A to 5C are a plan view and cross-sectional views illustratingone embodiment of a semiconductor device.

FIGS. 6A to 6C are a plan view and cross-sectional views illustratingone embodiment of a semiconductor device.

FIGS. 7A to 7E are cross-sectional views illustrating one embodiment ofa method for manufacturing a semiconductor device.

FIGS. 8A to 8C are a plan view and cross-sectional views illustratingone embodiment of a semiconductor device.

FIGS. 9A to 9C are plan views each illustrating one embodiment of asemiconductor device.

FIGS. 10A and 10B are a plan view and a cross-sectional viewillustrating one embodiment of a semiconductor device.

FIGS. 11A and 11B are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 12A and 12B are an equivalent circuit diagram and across-sectional view illustrating one embodiment of a semiconductordevice.

FIGS. 13A to 13C illustrate electronic appliances.

FIGS. 14A to 14C illustrate an electronic appliance.

FIG. 15 shows measurement results by SIMS.

FIG. 16 is a graph showing a relation between resistivity and whether ornot diluted hydrofluoric acid treatment is performed.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the invention disclosed in thisspecification will be described with reference to the accompanyingdrawings. Note that the invention disclosed in this specification is notlimited to the following description, and it is easily understood bythose skilled in the art that modes and details can be variously changedwithout departing from the spirit and the scope of the invention.Therefore, the invention disclosed in this specification is notconstrued as being limited to the description of the followingembodiments. Note that the ordinal numbers such as “first” and “second”in this specification are used for convenience and do not denote theorder of steps and the stacking order of layers. In addition, theordinal numbers in this specification do not denote particular nameswhich specify the present invention.

Embodiment 1

In this embodiment, an embodiment of a semiconductor device and amanufacturing method thereof will be described with reference to FIGS.1A to 1C, FIGS. 2A to 2E, FIGS. 3A to 3C, FIGS. 4A to 4C, and FIGS. 5Ato 5C. In this embodiment, a transistor including an oxide semiconductorfilm will be described as an example of the semiconductor device.

The transistor may have a single-gate structure in which one channelformation region is formed, a double-gate structure in which two channelformation regions are formed, or a triple-gate structure in which threechannel formation regions are formed. Alternatively, the transistor mayhave a dual-gate structure including two gate electrode layerspositioned above and below a channel formation region with a gateinsulating film provided therebetween.

A transistor 440 illustrated in FIGS. 1A to 1C is an example of atransistor which is one of bottom-gate transistors and is also referredto as an inverted staggered transistor. FIG. 1A is a plan view of thetransistor 440. FIG. 1B is a cross-sectional view taken along line X1-Y1in FIG. 1A. FIG. 1C is a cross-sectional view taken along line X2-Y2 inFIG. 1A.

The transistor 440 illustrated in FIGS. 1A to 1C includes a gateelectrode layer 401 which is provided over a substrate 400 having aninsulating surface, a gate insulating film 402 which is provided overthe gate electrode layer 401, an oxide semiconductor film 403 which hasan island shape and is provided over the gate insulating film 402, aninsulating layer 413 which is provided over the oxide semiconductor film403 and overlaps with the gate electrode layer 401, and a sourceelectrode layer 405 a and a drain electrode layer 405 b which are incontact with the oxide semiconductor film 403 and the insulating layer413. In addition, the transistor 440 may further include, as itscomponents, a base insulating film 436 which is provided over thesubstrate 400, and an interlayer insulating film 408 and a planarizationinsulating film 409 which cover the transistor 440.

In the transistor 440, a length w2 of each of the source electrode layer405 a and the drain electrode layer 405 b in the channel width directionis smaller than a length w1 of the oxide semiconductor film 403 in thechannel width direction, and part of a surface of the oxidesemiconductor film 403 is in contact with the interlayer insulating film408.

The oxide semiconductor film 403 is subjected to impurity-removingtreatment in the manufacturing process of the transistor 440; therefore,elements contained in an etching gas which is used for forming thesource electrode layer 405 a, the drain electrode layer 405 b, and thelike which are provided over and in contact with the oxide semiconductorfilm 403 hardly remain on the surface of the oxide semiconductor film403. Specifically, each of the chlorine concentration and the boronconcentration in the surface of the oxide semiconductor film 403 islower than or equal to 5×10¹⁸ atoms/cm³, preferably lower than or equalto 1×10¹⁸ atoms/cm³.

An oxide semiconductor used for the oxide semiconductor film 403contains at least indium (In). In particular, In and zinc (Zn) arepreferably contained. In addition, as a stabilizer for reducing thevariation in electric characteristics of a transistor using the oxidesemiconductor film, the oxide semiconductor preferably contains gallium(Ga) in addition to In and Zn. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer. Zirconium (Zr) ispreferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium(Lu) may be contained.

As the oxide semiconductor, for example, any of the following can beused: indium oxide; tin oxide; zinc oxide; a two-component metal oxidesuch as an In—Zn-based oxide, an In—Mg-based oxide, or an In—Ga-basedoxide; a three-component metal oxide such as an In—Ga—Zn-based oxide(also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-basedoxide, an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, anIn—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide,an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-basedoxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, anIn—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide,an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; a four-componentmetal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-basedoxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Note that here, for example, an “In—Ga—Zn—O-based oxide” means an oxidecontaining In, Ga, and Zn as its main component and there is noparticular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxidemay contain a metal element other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0; and m isnot an integer) may be used as the oxide semiconductor. Note that Mrepresents one or more metal elements selected from Ga, Fe, Mn, and Co.Alternatively, as the oxide semiconductor, a material expressed by achemical formula, In₂SnO₅(ZnO)_(n) (n>0 and n is a natural number) maybe used.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=2:2:1 (=2/5:2/5:1/5),In:Ga:Zn=1:3:2 (=1/6:1/2:1/3), or In:Ga:Zn=3:1:2 (=1/2:1/6:1/3), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn-based oxide with anatomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

For example, high mobility can be obtained relatively easily in the caseof using an In—Sn—Zn oxide. However, mobility can be increased byreducing the defect density in a bulk also in the case of using anIn—Ga—Zn-based oxide.

Note that for example, the expression “the composition of an oxideincluding In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide including In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example. The same applies to other oxides.

However, without limitation to the materials given above, a materialwith an appropriate composition may be used as the oxide semiconductordepending on needed semiconductor characteristics and electricalcharacteristics (e.g., mobility, threshold voltage, and variation). Inorder to obtain the required semiconductor characteristics, it ispreferable that the carrier concentration, the impurity concentration,the defect density, the atomic ratio between a metal element and oxygen,the interatomic distance, the density, and the like are set toappropriate values. Alternatively, oxide semiconductor films which havedifferent constitutions (typified by composition) may be stacked or maybe separately provided as the channel formation region and source anddrain regions as appropriate.

For example, the oxide semiconductor film 403 may be a stack in which afirst oxide semiconductor film, a second oxide semiconductor film, and athird oxide semiconductor film which have different compositions areprovided in this order. For example, the first oxide semiconductor filmand the third oxide semiconductor film may be formed using athree-component metal oxide, and the second oxide semiconductor film maybe formed using a two-component metal oxide. It is preferable that thefirst to third oxide semiconductor films are formed using materialswhich contain the same components. In the case where the materialscontaining the same components are used, the second oxide semiconductorfilm can be formed over the first oxide semiconductor film using acrystal layer of the first oxide semiconductor film as a seed;therefore, crystal growth of the second oxide semiconductor film can beeasily caused. The same applies to the third oxide semiconductor film.In addition, in the case where the materials containing the samecomponents are used, an interface property such as adhesion or electriccharacteristics is good.

Further, the first oxide semiconductor film, the second oxidesemiconductor film, and the third oxide semiconductor film may containthe same constituent elements and the composition of the constituentelements may be different among the first oxide semiconductor film, thesecond oxide semiconductor film, and the third oxide semiconductor film.For example, the first oxide semiconductor film and the third oxidesemiconductor film may have an atomic ratio of In:Ga:Zn=1:1:1, and thesecond oxide semiconductor film may have an atomic ratio ofIn:Ga:Zn=3:1:2. Alternatively, the first oxide semiconductor film andthe third oxide semiconductor film may have an atomic ratio ofIn:Ga:Zn=1:3:2, and the second oxide semiconductor film may have anatomic ratio of In:Ga:Zn=3:1:2. Further alternatively, the first oxidesemiconductor film may have an atomic ratio of In:Ga:Zn=1:3:2, thesecond oxide semiconductor film may have an atomic ratio ofIn:Ga:Zn=3:1:2, and the third oxide semiconductor film may have anatomic ratio of In:Ga:Zn=1:1:1.

The oxide semiconductor film 403 is in a single crystal state, apolycrystalline (also referred to as polycrystal) state, an amorphousstate, or the like.

The oxide semiconductor film 403 is preferably a c-axis alignedcrystalline oxide semiconductor (CAAC-OS) film.

The CAAC-OS film is not completely single crystal nor completelyamorphous. The CAAC-OS film is an oxide semiconductor film with acrystal-amorphous mixed phase structure where crystal portions areincluded in an amorphous phase. Note that in most cases, the crystalportion fits inside a cube whose one side is less than 100 nm. From anobservation image obtained with a transmission electron microscope(TEM), a boundary between an amorphous portion and a crystal portion inthe CAAC-OS film is not clear. Further, with the TEM, a grain boundaryin the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reductionin electron mobility, due to the grain boundary, is suppressed.

In each of the crystal portions included in the CAAC-OS film, a c-axisis aligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, triangular or hexagonal atomic arrangement which is seenfrom the direction perpendicular to the a-b plane is formed, and metalatoms are arranged in a layered manner or metal atoms and oxygen atomsare arranged in a layered manner when seen from the directionperpendicular to the c-axis. Note that, among crystal portions, thedirections of the a-axis and the b-axis of one crystal portion may bedifferent from those of another crystal portion. In this specification,a simple term “perpendicular” includes a range from 85° to 95°. Inaddition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal portions is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal portions in the vicinityof the surface of the oxide semiconductor film is higher than that inthe vicinity of the surface where the oxide semiconductor film is formedin some cases. Further, when an impurity is added to the CAAC-OS film,the crystal portion in a region to which the impurity is added becomesamorphous in some cases.

Since the c-axes of the crystal portions included in the CAAC-OS filmare aligned in the direction parallel to a normal vector of a surfacewhere the CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note thatwhen the CAAC-OS film is formed, the direction of the c-axis of thecrystal portion is the direction parallel to a normal vector of thesurface where the CAAC-OS film is formed or a normal vector of thesurface of the CAAC-OS film. The crystal portion is formed by depositionor by performing treatment for crystallization such as heat treatmentafter deposition.

With use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light can be reduced. Thus, the transistor has highreliability.

Note that part of oxygen included in the oxide semiconductor film may besubstituted with nitrogen.

In an oxide semiconductor having a crystal portion such as the CAAC-OS,defects in the bulk can be further reduced and when the surface flatnessof the oxide semiconductor is improved, mobility higher than that of anoxide semiconductor in an amorphous state can be obtained. In order toimprove the surface flatness, the oxide semiconductor is preferablyformed over a flat surface. Specifically, the oxide semiconductor may beformed over a surface with an average surface roughness (Ra) of lessthan or equal to 1 nm, preferably less than or equal to 0.3 nm, furtherpreferably less than or equal to 0.1 nm.

The average surface roughness (Ra) is obtained by expanding, into threedimensions, arithmetic mean surface roughness that is defined by JIS B0601:2001 (ISO4287:1997) so as to be able to apply to a curved surface,and can be expressed as an “average value of the absolute values ofdeviations from a reference surface to a designated surface” and isdefined by the following formula:

$\begin{matrix}{{Ra} = {\frac{1}{S_{0}}{\int_{y_{1}}^{y_{2}}{\int_{x_{1}}^{x_{2}}{{{{f\left( {x,y} \right)} - Z_{0}}}\ {\mathbb{d}x}\ {\mathbb{d}y}}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here, the specific surface is a surface which is a target of roughnessmeasurement, and is a quadrilateral region which is specified by fourpoints represented by the coordinates (x₁, y₁, f(x₁, y₁)), (x₁, y₂,f(x₁, y₂)), (x₂, y₁, f(x₂, y₁)), and (x₂, y₂, f(x₂, y₂)). Moreover, S₀represents the area of a rectangle which is obtained by projecting thespecific surface on the xy plane, and Z₀ represents the height of thereference surface (the average height of the specific surface). Ra canbe measured using an atomic force microscope (AFM).

Note that, since the transistor 440 described in this embodiment is abottom-gate transistor, the substrate 400, the gate electrode layer 401,and the gate insulating film 402 are positioned below the oxidesemiconductor film 403. Accordingly, planarization treatment such as CMPtreatment may be performed after the formation of the gate electrodelayer 401 and the gate insulating film 402 to obtain the above flatsurface.

The oxide semiconductor film 403 has a thickness greater than or equalto 1 nm and less than or equal to 30 nm (preferably greater than orequal to 5 nm and less than or equal to 10 nm) and can be formed by asputtering method, a molecular beam epitaxy (MBE) method, a CVD method,a pulsed laser deposition method, an atomic layer deposition (ALD)method, or the like as appropriate. The oxide semiconductor film 403 maybe formed using a sputtering apparatus which performs deposition withsurfaces of a plurality of substrates set substantially perpendicular toa surface of a sputtering target (a columnar plasma sputtering system).

Next, an example of a method for manufacturing the transistor 440 shownin FIGS. 1A to 1C is described with reference to FIGS. 2A to 2E.

First, the base insulating film 436 is formed over the substrate 400having an insulating surface.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, silicon carbide, or the like; a compound semiconductorsubstrate of silicon germanium or the like; an SOI substrate; or thelike can be used as the substrate 400, or the substrate provided with asemiconductor element can be used as the substrate 400.

The semiconductor device may be manufactured using a flexible substrateas the substrate 400. To manufacture a flexible semiconductor device,the transistor 440 including the oxide semiconductor film 403 may bedirectly formed over a flexible substrate; or alternatively, thetransistor 440 including the oxide semiconductor film 403 may be formedover a substrate, and then may be separated and transferred to aflexible substrate. Note that in order to separate the transistor 440including the oxide semiconductor film from the manufacturing substrateand transfer it to the flexible substrate, a separation layer may beprovided between the manufacturing substrate and the transistor 440.

The base insulating film 436 can be formed by a plasma CVD method, asputtering method, or the like using an oxide insulating material suchas silicon oxide, silicon oxynitride, aluminum oxide, aluminumoxynitride, hafnium oxide, gallium oxide, or the like; a nitrideinsulating material such as silicon nitride, silicon nitride oxide, analuminum nitride, aluminum nitride oxide, or the like; or a mixedmaterial thereof. Note that the base insulating film 436 is notnecessarily provided.

The substrate 400 (or the substrate 400 and the base insulating film436) may be subjected to heat treatment. For example, the heat treatmentmay be performed with a gas rapid thermal annealing (GRTA) apparatus, inwhich heat treatment is performed using a high-temperature gas, at 650°C. for 1 minute to 5 minutes. As the high-temperature gas for GRTA, aninert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas like argon, is used.Alternatively, the heat treatment may be performed with an electricfurnace at 500° C. for 30 minutes to 1 hour.

Then, a conductive film is formed over the base insulating film 436 andetched to form the gate electrode layer 401 (including a wiring formedusing the same layer). Note that the etching of the conductive film maybe performed using either dry etching or wet etching, or using both dryetching and wet etching.

The gate electrode layer 401 can be formed using a metal material suchas molybdenum, titanium, tantalum, tungsten, aluminum, copper, chromium,neodymium, or scandium or an alloy material which contains any of thesematerials as its main component. A semiconductor film which is dopedwith an impurity element such as phosphorus and is typified by apolycrystalline silicon film, or a silicide film of nickel silicide orthe like can also be used as the gate electrode layer 401. The gateelectrode layer 401 has either a single-layer structure or astacked-layer structure.

The gate electrode layer 401 can also be formed using a conductivematerial such as indium oxide-tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium oxide-zinc oxide, or indium tin oxide to which siliconoxide is added. It is also possible that the gate electrode layer 401has a stacked structure of the above conductive material and the abovemetal material.

As the gate electrode layer 401, which is in contact with the gateinsulating film 402, a metal oxide containing nitrogen, specifically, anIn—Ga—Zn—O film containing nitrogen, an In—Sn—O film containingnitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O filmcontaining nitrogen, a Sn—O film containing nitrogen, an In—O filmcontaining nitrogen, or a metal nitride (e.g., InN or SnN) film can beused. These films each have a work function of 5 eV or higher,preferably 5.5 eV or higher, which enables the threshold voltage of thetransistor to take a positive value when used as the gate electrodelayer, so that a switching element of what is called normally-off typecan be realized.

In this embodiment, a 100-nm-thick tungsten film is formed by asputtering method.

After the formation of the gate electrode layer 401, the substrate 400and the gate electrode layer 401 may be subjected to heat treatment. Forexample, the heat treatment may be performed with a GRTA apparatus at650° C. for 1 minute to 5 minutes. Alternatively, the heat treatment maybe performed with an electric furnace at 500° C. for 30 minutes to 1hour.

Next, the gate insulating film 402 is formed over the gate electrodelayer 401.

To improve the coverage with the gate insulating film 402, planarizationtreatment may be performed on a surface of the gate electrode layer 401.It is preferable that the flatness of the surface of the gate electrodelayer 401 is good particularly when the thickness of the gate insulatingfilm 402 is small.

The gate insulating film 402 can be formed to have a thickness greaterthan or equal to 1 nm and less than or equal to 20 nm by a sputteringmethod, an MBE method, a CVD method, a pulsed laser deposition method,an ALD method, or the like as appropriate. The gate insulating film 402may be formed with a sputtering apparatus which performs deposition onsurfaces of a plurality of substrates set substantially perpendicular toa surface of a sputtering target.

As a material of the gate insulating film 402, a silicon oxide film, agallium oxide film, an aluminum oxide film, a silicon nitride film, asilicon oxynitride film, an aluminum oxynitride film, or a siliconnitride oxide film can be used.

When the gate insulating film 402 is formed using a high-k material suchas hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0,y>0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen isadded, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)), or lanthanum oxide,gate leakage current can be reduced. Further, the gate insulating film402 may have either a single-layer structure or a stacked-layerstructure.

It is preferable that the gate insulating film 402 includes oxygen in aportion which is in contact with the oxide semiconductor film 403. Inparticular, the gate insulating film 402 preferably contains a largeamount of oxygen which exceeds at least the stoichiometric compositionin the film (bulk). For example, in the case where a silicon oxide filmis used as the gate insulating film 402, the composition formula isSiO_(2+α), (α>0).

When the gate insulating film 402 containing much (excess) oxygen, whichserves as an oxygen supply source, is provided so as to be in contactwith the oxide semiconductor film 403, oxygen can be supplied from thegate insulating film 402 to the oxide semiconductor film 403. Heattreatment may be performed in the state where the oxide semiconductorfilm 403 and the gate insulating film 402 are at least partly in contactwith each other to supply oxygen to the oxide semiconductor film 403.

By supply of oxygen to the oxide semiconductor film 403, oxygenvacancies in the film can be repaired. Further, the gate insulating film402 is preferably formed in consideration of the size of a transistor tobe formed and the step coverage with the gate insulating film 402.

In this embodiment, a 200-nm-thick silicon oxynitride film is formed bya high-density plasma CVD method.

After the formation of the gate insulating film 402, the substrate 400,the gate electrode layer 401, and the gate insulating film 402 may besubjected to heat treatment. For example, the heat treatment may beperformed with a GRTA apparatus at 650° C. for 1 minute to 5 minutes.Alternatively, the heat treatment may be performed with an electricfurnace at 500° C. for 30 minutes to 1 hour.

Next, the oxide semiconductor film 403 is formed over the gateinsulating film 402.

In order that hydrogen or water will not enter the oxide semiconductorfilm 403 as much as possible in the formation step of the oxidesemiconductor film 403, it is preferable to heat the substrate providedwith the gate insulating film 402 in a preheating chamber in asputtering apparatus as a pretreatment for formation of the oxidesemiconductor film 403 so that impurities such as hydrogen and moistureadsorbed onto the substrate and the gate insulating film 402 areeliminated and exhausted. As an exhaustion unit provided in thepreheating chamber, a cryopump is preferable.

Planarizing treatment may be performed on the region of the gateinsulating film 402 which is in contact with the oxide semiconductorfilm 403. As the planarization treatment, polishing treatment (e.g.,chemical mechanical polishing (CMP) method), dry-etching treatment, orplasma treatment can be used, though there is no particular limitationon the planarizing treatment.

As plasma treatment, reverse sputtering in which an argon gas isintroduced and plasma is generated can be performed. The reversesputtering is a method in which voltage is applied to a substrate sidewith use of an RF power source in an argon atmosphere and plasma isgenerated in the vicinity of the substrate so that a substrate surfaceis modified. Note that instead of an argon atmosphere, a nitrogenatmosphere, a helium atmosphere, an oxygen atmosphere, or the like maybe used. The reverse sputtering can remove particle substances (alsoreferred to as particles or dust) attached to the top surface of thegate insulating film 402.

As the planarization treatment, polishing treatment, dry etchingtreatment, or plasma treatment may be performed plural times, or thesetreatments may be performed in combination. In the case where thetreatments are combined, the order of steps is not particularly limitedand may be set as appropriate depending on the roughness of the surfaceof the gate insulating film 402.

The oxide semiconductor film 403 is preferably deposited under acondition such that much oxygen is contained (for example, by asputtering method in an atmosphere where the proportion of oxygen is100%) so as to be a film containing much oxygen (preferably including aregion containing excessive oxygen as compared to the stoichiometriccomposition of the oxide semiconductor in a crystalline state).

In this embodiment, a 35-nm-thick In—Ga—Zn-based oxide film (IGZO film)is formed as the oxide semiconductor film 403 by a sputtering methodusing a sputtering apparatus including an AC power supply device. Inthis embodiment, an In—Ga—Zn-based oxide target having an atomic ratioof In:Ga:Zn=1:1:1 (=1/3:1/3:1/3) is used. Note that the depositioncondition is as follows: the atmosphere is an atmosphere of oxygen andargon (oxygen flow rate: 50%), the pressure is 0.6 Pa, the power of thepower source is 5 kW, and the substrate temperature is 170° C. Thedeposition rate under the deposition condition is 16 nm/min.

It is preferable that a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed is used as asputtering gas for the deposition of the oxide semiconductor film 403.

The substrate is held in a deposition chamber kept under reducedpressure. Then, a sputtering gas from which impurities such as hydrogenand moisture are sufficiently removed is introduced into the depositionchamber from which remaining moisture is being removed, and the oxidesemiconductor film 403 is formed over the substrate 400 with the use ofthe target. In order to remove moisture remaining in the depositionchamber, an entrapment vacuum pump such as a cryopump, an ion pump, or atitanium sublimation pump is preferably used. As an exhaustion unit, aturbo molecular pump to which a cold trap is added may be used. In thedeposition chamber which is evacuated with the cryopump, for example, ahydrogen atom, a compound containing a hydrogen atom, such as water(H₂O), (further preferably, also a compound containing a carbon atom),and the like are removed, whereby the concentration of impurities in theoxide semiconductor film 403 formed in the deposition chamber can bereduced.

It is preferable to form the gate insulating film 402 and the oxidesemiconductor film 403 in succession so as not to expose the gateinsulating film 402 to the air. Forming the gate insulating film 402 andthe oxide semiconductor film 403 in succession so as not to expose thegate insulating film 402 to the air can prevent impurities such ashydrogen and moisture from being adsorbed to the surface of the gateinsulating film 402.

In the case where a CAAC-OS film is used as the oxide semiconductor film403, for example, the CAAC-OS film is formed by a sputtering method witha polycrystalline oxide semiconductor sputtering target. When ionscollide with the sputtering target, a crystal region included in thesputtering target may be separated from the target along an a-b plane;in other words, a sputtered particle having a plane parallel to an a-bplane (flat-plate-like sputtered particle or pellet-like sputteredparticle) may flake off from the sputtering target. In that case, theflat-plate-like sputtered particle reaches a substrate while maintainingtheir crystal state, whereby a crystal state of the sputtering target istransferred to the substrate. As a result, the CAAC-OS film can beformed.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the amount of impurities entering the CAAC-OS film duringthe deposition, the crystal state can be prevented from being broken bythe impurities. For example, impurities (e.g., hydrogen, water, carbondioxide, or nitrogen) which exist in the deposition chamber may bereduced. Furthermore, impurities in a deposition gas may be reduced.Specifically, a deposition gas whose dew point is −80° C. or lower,preferably −100° C. or lower is used.

By increasing a substrate heating temperature during the deposition,migration of a sputtered particle is likely to occur after the sputteredparticle reaches a substrate surface. Specifically, the substrateheating temperature during the deposition is set to higher than or equalto 100° C. and lower than or equal to 740° C., preferably higher than orequal to 200° C. and lower than or equal to 500° C. By increasing thesubstrate heating temperature during the deposition, when theflat-plate-like sputtered particle reaches the substrate, migrationoccurs on the substrate surface, so that a flat plane of theflat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas is increased and the power is optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is greater than or equal to 30 vol %, preferably 100 vol%.

As an example of the sputtering target, an In—Ga—Zn—O compound target isdescribed below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made bymixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined ratio, applying pressure, and performing heat treatment ata temperature higher than or equal to 1000° C. and lower than or equalto 1500° C. Note that X, Y, and Z are each a given positive number.Here, the predetermined molar ratio of InO_(X) powder to GaO_(Y) powderand ZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3,or 3:1:2. The kinds of powder and the molar ratio for mixing powder maybe determined as appropriate depending on the desired sputtering target.

The oxide semiconductor film 403 can be formed by processing an oxidesemiconductor film into an island shape by a photolithography process.

A resist mask for forming the oxide semiconductor film 403 having anisland shape may be formed by an inkjet method. Formation of the resistmask by an inkjet method needs no photomask; thus, manufacturing costcan be reduced.

Etching of the oxide semiconductor film may be dry etching, wet etching,or both dry etching and wet etching. As an etchant used for wet etchingof the oxide semiconductor film, for example, a mixed solution ofphosphoric acid, acetic acid, and nitric acid, or the like can be used.Alternatively, ITO-07N (produced by KANTO CHEMICAL CO., INC.) may beused. Further alternatively, etching may be performed by a dry etchingemploying an inductively coupled plasma (ICP) etching method.

Further, heat treatment may be performed on the oxide semiconductor film403 in order to remove excess hydrogen (including water and a hydroxylgroup) (to perform dehydration or dehydrogenation treatment). Thetemperature of the heat treatment is higher than or equal to 300° C. andlower than or equal to 700° C., or lower than the strain point of thesubstrate. The heat treatment can be performed under reduced pressure, anitrogen atmosphere, or the like.

When a crystalline oxide semiconductor film is used as the oxidesemiconductor film 403, heat treatment for crystallization may beperformed.

In this embodiment, the substrate is introduced into an electricfurnace, which is one of heat treatment apparatuses, and the oxidesemiconductor film 403 is subjected to heat treatment at 450° C. in anitrogen atmosphere for 1 hour and further at 450° C. in an atmosphereof nitrogen and oxygen for 1 hour.

Further, a heat treatment apparatus used is not limited to an electricfurnace, and a device for heating a process object by heat conduction orheat radiation from a heating element such as a resistance heatingelement may be alternatively used. For example, an RTA (rapid thermalanneal) apparatus such as a GRTA (gas rapid thermal anneal) apparatus oran LRTA (lamp rapid thermal anneal) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed as follows.The substrate is put in an inert gas heated at high temperature of 650°C. to 700° C., is heated for several minutes, and is taken out of theinert gas.

Note that in heat treatment, it is preferable that moisture, hydrogen,and the like are not contained in nitrogen or a rare gas such as helium,neon, or argon. The purity of nitrogen or the rare gas such as helium,neon, or argon which is introduced into the heat treatment apparatus isset to preferably 6N (99.9999%) or higher, further preferably 7N(99.99999%) or higher (that is, the impurity concentration is preferably1 ppm or lower, further preferably 0.1 ppm or lower).

In addition, after the oxide semiconductor film 403 is heated by theheat treatment, a high-purity oxygen gas, a high-purity N₂O gas, orultra dry air (the moisture amount is less than or equal to 20 ppm (−55°C. by conversion into a dew point), preferably less than or equal to 1ppm, further preferably less than or equal to 10 ppb according to themeasurement with a dew point meter of a cavity ring down laserspectroscopy (CRDS) system) may be introduced into the same furnace. Itis preferable that water, hydrogen, or the like is not contained in theoxygen gas or the dinitrogen monoxide gas. Alternatively, the purity ofthe oxygen gas or the dinitrogen monoxide gas which is introduced intothe heat treatment apparatus is preferably 6N or higher, furtherpreferably 7N or higher (i.e., the impurity concentration in the oxygengas or the dinitrogen monoxide gas is preferably 1 ppm or lower, furtherpreferably 0.1 ppm or lower). The oxygen gas or the dinitrogen monoxidegas acts to supply oxygen that is a main component of the oxidesemiconductor and that is reduced by the step for removing an impurityfor the dehydration or dehydrogenation, so that the oxide semiconductorfilm 403 can be a highly purified, i-type (intrinsic) oxidesemiconductor film.

Note that the timing of the heat treatment for dehydration ordehydrogenation may be after formation of the oxide semiconductor filmor after formation of the oxide semiconductor film 403 having an islandshape.

The heat treatment for dehydration or dehydrogenation may be performedplural times and may be combined with another heat treatment.

When the heat treatment for dehydration or dehydrogenation is performedin the state where the gate insulating film 402 is covered with theoxide semiconductor film 403 which has not been processed into anisland-shaped shape, oxygen contained in the gate insulating film 402can be prevented from being released by the heat treatment, which ispreferable.

Further or alternatively, oxygen (which includes at least one of anoxygen radical, an oxygen atom, and an oxygen ion) may be introduced tothe oxide semiconductor film 403 which has been subjected to thedehydration or dehydrogenation treatment to supply oxygen to the oxidesemiconductor film.

The dehydration or dehydrogenation treatment may be accompanied byelimination of oxygen which is a main component material of an oxidesemiconductor to lead to a reduction in oxygen. An oxygen vacancy existsin a portion where oxygen is eliminated in an oxide semiconductor film,and a donor level which leads to a change in the electriccharacteristics of a transistor is formed owing to the oxygen vacancy.

Therefore, it is preferable that oxygen (which includes at least one ofan oxygen radical, an oxygen atom, and an oxygen ion) is added to theoxide semiconductor film which has been subjected to the dehydration ordehydrogenation treatment to supply oxygen to the oxide semiconductorfilm. By supply of oxygen to the oxide semiconductor film, oxygenvacancies in the film can be repaired.

Oxygen which is introduced to the dehydrated or dehydrogenated oxidesemiconductor film 403 to supply oxygen to the film can highly purifythe oxide semiconductor film 403 and make the film an i-type(intrinsic). Variation in electric characteristics of the transistorincluding the i-type (intrinsic) oxide semiconductor film 403 issuppressed, and the transistor is electrically stable.

Oxygen can be introduced by an ion implantation method, an ion dopingmethod, a plasma immersion ion implantation method, plasma treatment, orthe like.

In the case where oxygen is introduced into the oxide semiconductor film403, oxygen may be directly introduced into the oxide semiconductor film403, or may be introduced into the oxide semiconductor film 403 throughother films such as an insulating layer 413. An ion implantation method,an ion doping method, a plasma immersion ion implantation method, or thelike may be employed for the introduction of oxygen through anotherfilm, whereas plasma treatment or the like can also be employed for theintroduction of oxygen directly into the oxide semiconductor film 403which is exposed.

The introduction of oxygen to the oxide semiconductor film 403 ispreferably performed anytime after dehydration or dehydrogenationtreatment is performed thereon, but the timing is not limited thereto.Further, oxygen may be introduced plural times into the dehydrated ordehydrogenated oxide semiconductor film 403.

Further, it is preferable that the oxide semiconductor film provided inthe transistor includes a region containing excessive oxygen as comparedto the stoichiometric composition of the oxide semiconductor in acrystalline state. In that case, the oxygen content is preferably higherthan that in the stoichiometric ratio of the oxide semiconductor.Alternatively, the oxygen content is higher than that of the oxidesemiconductor in a single crystal state. In some cases, oxygen may existbetween lattices of the oxide semiconductor.

By removing hydrogen or moisture from the oxide semiconductor to highlypurify the oxide semiconductor so as not to contain impurities as muchas possible, and supplying oxygen to repair oxygen vacancies therein,the oxide semiconductor can be turned into an i-type (intrinsic) oxidesemiconductor or a substantially i-type (intrinsic) oxide semiconductor.This enables the Fermi level (E_(f)) of the oxide semiconductor to be atthe same level as the intrinsic Fermi level (E_(i)) thereof.Accordingly, by using the oxide semiconductor film for a transistor,fluctuation in the threshold voltage Vth of the transistor due to anoxygen vacancy and a shift of the threshold voltage ΔVth can be reduced.

Next, the insulating layer 413 is formed over the channel formationregion of the oxide semiconductor film 403 which overlaps with the gateelectrode layer 401 (see FIG. 2A).

The insulating layer 413 can be formed by etching of an insulating filmwhich is deposited by a plasma CVD method or a sputtering method. As theinsulating layer 413, a single layer or a stack of one or more inorganicinsulating films, typical examples of which include a silicon oxidefilm, a silicon oxynitride film, an aluminum oxide film, an aluminumoxynitride film, a hafnium oxide film, a gallium oxide film, a siliconnitride film, an aluminum nitride film, a silicon nitride oxide film,and an aluminum nitride oxide film can be used.

When the insulating layer 413 in contact with the oxide semiconductorfilm 403 (or a film in contact with the oxide semiconductor film 403 inthe case where the insulating layer 413 has a stacked-layer structure)contains a large amount of oxygen, the insulating layer 413 (or the filmin contact with the oxide semiconductor film 403) can favorably functionas a supply source which supplies oxygen to the oxide semiconductor film403.

In this embodiment, a 200-nm-thick silicon oxide film is formed as theinsulating layer 413 by a sputtering method. The silicon oxide film isselectively etched to form the insulating layer 413 whosecross-sectional shape is a trapezoid or a triangle and in which a taperangle of a bottom edge portion of the cross-sectional shape is less thanor equal to 60°, preferably less than or equal to 45°, furtherpreferably less than or equal to 30°. Note that the planar shape of theinsulating layer 413 is a rectangle. In this embodiment, a resist maskis formed over the silicon oxide film through a photolithography processand selective etching is performed, so that the taper angle of thebottom edge portion of the insulating layer 413 is approximately 30°.

After the formation of the insulating layer 413, heat treatment may beperformed. In this embodiment, heat treatment is performed at 300° C.for 1 hour in a nitrogen atmosphere.

Next, a conductive film 445 to be a source electrode layer and a drainelectrode layer (including a wiring formed using the same layer) isformed over the gate electrode layer 401, the gate insulating film 402,the oxide semiconductor film 403, and the insulating layer 413 (see FIG.2B).

The conductive film 445 is formed of a material that can withstand heattreatment performed later. As the conductive film 445 used for formingthe source electrode layer and the drain electrode layer, for example, ametal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W, a metal nitride film containing any of the above elements as itscomponent (e.g., a titanium nitride film, a molybdenum nitride film, ora tungsten nitride film), or the like can be used. A metal film having ahigh melting point such as Ti, Mo, W, or the like or a metal nitridefilm of any of these elements (a titanium nitride film, a molybdenumnitride film, and a tungsten nitride film) may be stacked on one of orboth of a lower side and an upper side of a metal film of Al, Cu, or thelike. Alternatively, the conductive film 445 used for forming the sourceelectrode layer and the drain electrode layer may be formed using aconductive metal oxide. Examples of the conductive metal oxide areindium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indiumoxide-tin oxide (In₂O₃—SnO₂, abbreviated to ITO), indium oxide-zincoxide (In₂O₃—ZnO), and any of these metal oxide materials containingsilicon oxide.

Resist masks 448 a and 448 b are formed over the conductive film 445through a photolithography process and selective etching is performed,so that the source electrode layer 405 a and the drain electrode layer405 b are formed (see FIG. 2C). After the source electrode layer 405 aand the drain electrode layer 405 b are formed, the resist masks areremoved.

A gas 442 containing a halogen element is used for the etching of theconductive film 445. A gas containing sulfur hexafluoride (SF₆), carbontetrafluoride (CF₄), chlorine (Cl₂), boron trichloride (BCl₃), silicontetrachloride (SiCl₄), carbon tetrachloride (CCl₄), or the like can beused as the gas 442 containing the halogen element.

A reactive ion etching (RIE) method or an ICP etching method can be usedas the etching method. In order to etch the film into desired shapes,the etching conditions (the amount of electric power applied to acoil-shaped electrode, the amount of electric power applied to anelectrode on a substrate side, the temperature of the electrode on thesubstrate side, or the like) is adjusted as appropriate.

In this embodiment, a stack of a 100-nm-thick titanium film, a400-nm-thick aluminum film, and a 100-nm-thick titanium film is formedas the conductive film 445 by a sputtering method. As the etching of theconductive film 445, the stack of the titanium film, the aluminum film,and the titanium film is etched by a dry etching method, whereby thesource electrode layer 405 a and the drain electrode layer 405 b areformed.

In this embodiment, the upper titanium film and the aluminum film areetched under a first etching condition and then the lower titanium filmis etched under a second etching condition. The first etching conditionis as follows: an etching gas (BCl₃:Cl₂=750 sccm:150 sccm) is used, thebias power is 1500 W, the power of an ICP power source is 0 W, and thepressure is 2.0 Pa. The second etching condition is as follows: anetching gas (BCl₃:Cl₂=700 sccm:100 sccm) is used, the bias power is 750W, the power of the ICP power source is 0 W, and the pressure is 2.0 Pa.

One of edge portions of the source electrode layer 405 a in the channellength direction is located over a top surface or a side surface of theinsulating layer 413, and one of edge portions of the drain electrodelayer 405 b in the channel length direction is located over the topsurface or a side surface of the insulating layer 413. In addition, asillustrated in FIGS. 1A and 1C, the length w1 of the oxide semiconductorfilm 403 in the channel width direction is larger than the length w2 ofeach of the source electrode layer 405 a and the drain electrode layer405 b in the channel width direction. Therefore, edge portions of thesource electrode layer 405 a in the channel width direction are locatedover the oxide semiconductor film 403, and edge portions of the drainelectrode layer 405 b in the channel width direction are located overthe oxide semiconductor film 403. Therefore, part of the oxidesemiconductor film 403 (regions which do not overlap with any of thesource electrode layer 405 a, the drain electrode layer 405 b, or theinsulating layer 413) is exposed to a gas 442 containing a halogenelement at the time when the conductive film 445 is etched to form thesource electrode layer 405 a and the drain electrode layer 405 b.

When the oxide semiconductor film 403 is exposed to the gas 442containing the halogen element and the halogen element contained in theetching gas remains on the surface of the oxide semiconductor film 403,oxygen in the oxide semiconductor film 403 might be extracted by thehalogen element, so that oxygen vacancies are formed in the periphery ofthe interface of the oxide semiconductor film 403 in some cases. Inaddition, an element (e.g., boron) which is not a halogen element and iscontained in the etching gas containing the halogen element might alsocause the back channel of the oxide semiconductor film 403 to have lowerresistance (n-type conductivity).

Therefore, after the source electrode layer 405 a and the drainelectrode layer 405 b are formed, impurities on the surface of the oxidesemiconductor film 403 and on the periphery thereof (here, the elementscontained in the etching gas) are removed (see FIG. 2D). Theimpurity-removing treatment can be performed by treatment with asolution or plasma treatment using oxygen or dinitrogen monoxide. As thesolution, water, an alkaline solution (e.g., a developing solution or anammonia hydrogen peroxide mixture), an acid solution (e.g., dilutedhydrofluoric acid (hydrofluoric acid which is diluted at a ratio of1:100 (0.5% hydrofluoric acid), preferably at a ratio of 1:10³ or moreand 1:10⁵ or less)) can be preferably used. In addition, as theimpurity-removing treatment, the above treatments may be performed incombination. For example, plasma treatment using oxygen may be performedand then treatment using diluted hydrofluoric acid may be performed.

FIG. 15 shows measurement results by secondary ion mass spectrometry(SIMS) in which the chlorine concentration in an oxide semiconductorfilm of a transistor which was manufactured without cleaning treatmentwas measured. The sample transistor has the same structure as thetransistor 440 of this embodiment except that the cleaning treatment isnot performed, and the sample transistor was manufactured using the samematerial and method as the transistor 440. Note that a region where aninsulating layer functioning as a channel protective film was not formedwas measured. In the region, a silicon oxynitride film (with a thicknessof 400 nm) serving as a protective insulating film, an IGZO film servingas an oxide semiconductor film, and a silicon oxynitride film serving asa gate insulating film are stacked toward the depth direction. Themeasurement was performed from the protective insulating film toward thedepth direction.

FIG. 15 shows that the chlorine concentration in the IGZO film servingas the oxide semiconductor film is higher than 1×10¹⁹ atoms/cm³, whichindicates that the oxide semiconductor film contains chlorine.

In this embodiment, the oxide semiconductor film 403 which is exposed tothe etching gas is subjected to the impurity-removing treatment afterthe source electrode layer 405 a and the drain electrode layer 405 b areformed, whereby the elements (e.g., chlorine and boron) contained in theetching gas can be removed. For example, in the surface of the oxidesemiconductor film 403 which has been subjected to the impurity-removingtreatment, the chlorine concentration and the boron concentration eachcan be lower than or equal to 5×10¹⁸ atoms/cm³ (preferably lower than orequal to 1×10¹⁸ atoms/cm³).

Through the above-described process, the transistor 440 of thisembodiment can be manufactured (see FIG. 2E).

Note that the interlayer insulating film 408 covering the transistor 440and the planarization insulating film 409 for reducing surfaceunevenness which is caused by the transistor 440 may be provided.

The interlayer insulating film 408 can be formed using the same materialand method as the insulating layer 413. For example, a 400-nm-thicksilicon oxynitride film is formed as the interlayer insulating film 408by a CVD method. In addition, heat treatment may be performed after theinterlayer insulating film 408 is formed. For example, the heattreatment is performed at 300° C. for 1 hour in a nitrogen atmosphere.

Further, a dense inorganic insulating film may be provided as theinterlayer insulating film 408. For example, an aluminum oxide film isformed as the interlayer insulating film 408 by a sputtering method.When the aluminum oxide film has high density (the film density ishigher than or equal to 3.2 g/cm³, preferably higher than or equal to3.6 g/cm³), the transistor 440 can have stable electric characteristics.The film density can be measured by Rutherford backscatteringspectrometry (RBS) or X-ray reflectometry (XRR).

An aluminum oxide film which can function as a protective insulatingfilm of the transistor 440 has a high shielding effect (blocking effect)of preventing penetration of both oxygen and impurities such as hydrogenand moisture.

Therefore, in and after the manufacturing process, the aluminum oxidefilm prevents the entry of impurities such as hydrogen and moisture,which causes a change, into the oxide semiconductor film 403 and releaseof oxygen, which is a main component material of the oxidesemiconductor, from the oxide semiconductor film 403.

As the planarization insulating film 409, an organic material such as apolyimide-based resin, an acrylic-based resin, or abenzocyclobutene-based resin can be used. Other than such organicmaterials, it is also possible to use a low-dielectric constant material(a low-k material) or the like. Note that the planarization insulatingfilm may be formed by stacking a plurality of insulating films formedfrom these materials.

For example, a 1500-nm-thick acrylic resin film may be formed as theplanarization insulating film 409. The acrylic resin film can be formedin such a manner that an acrylic resin is applied by a coating methodand then baked (e.g., at 250° C. in a nitrogen atmosphere for 1 hour).

Heat treatment may be performed after the planarization insulating film409 is formed. For example, heat treatment is performed at 250° C. for 1hour in a nitrogen atmosphere.

In this manner, the heat treatment may be performed after the transistor440 is formed. Further, the heat treatment may be performed more thanonce.

As described above, the impurity-removing treatment for removing theelements contained in the etching gas can prevent oxygen from beingextracted from the surface of the oxide semiconductor film 403 and theperiphery thereof by the halogen element contained in the etching gas,or can prevent a reduction in the resistance of the back channel of theoxide semiconductor film 403 (prevent the back channel from havingn-type conductivity) by the element which is not a halogen element andcontained in the etching gas. As a result, the use of the oxidesemiconductor film 403 enables the transistor 440 to have stableelectrical characteristics and high reliability.

FIGS. 3A to 3C illustrate another structure of a transistor according tothis embodiment. FIG. 3A is a plan view of a transistor 450. FIG. 3B isa cross-sectional view taken along line X3-Y3 in FIG. 3A. FIG. 3C is across-sectional view taken along line X4-Y4 in FIG. 3A.

The transistor 450 illustrated in FIGS. 3A to 3C is an example in whichregions of the oxide semiconductor film 403 which are exposed from thesource electrode layer 405 a and the drain electrode layer 405 b areetched by the impurity-removing treatment, so that the thicknesses ofthe regions are reduced. For example, when the IGZO film is processedusing hydrofluoric acid which is diluted at a ratio of 1:10³ (0.05%hydrofluoric acid), the thickness is reduced by 1 nm to 3 nm per second;when the IGZO film is processed using hydrofluoric acid which is dilutedat a ratio of 2:10⁵ (0.0025% diluted hydrofluoric acid), the thicknessis reduced by approximately 0.1 nm per second.

In the oxide semiconductor film 403 included in the transistor 450, aregion which overlaps with the insulating layer 413, the sourceelectrode layer 405 a, or the drain electrode layer 405 b has a largerthickness than the regions which do not overlap with any of theinsulating layer 413, the source electrode layer 405 a, or the drainelectrode layer 405 b. The transistor 450 can have the same structure asthe transistor 440 except for the thickness of the oxide semiconductorfilm 403.

In addition, the insulating layer 413 provided over the oxidesemiconductor film 403 may also be processed by plasma treatment usingan etching gas containing a halogen element. In this case,impurity-removing treatment is preferably performed after the etchingtreatment for forming the insulating layer 413. A method which issimilar to the impurity-removing treatment performed after the formationof the source electrode layer 405 a and the drain electrode layer 405 bmay be applied to the impurity-removing treatment.

In the case where first impurity-removing treatment is performed afterthe formation of the insulating layer 413 and second impurity-removingtreatment is performed after the formation of the source electrode layer405 a and the drain electrode layer 405 b, part of the oxidesemiconductor film 403 may be etched depending on conditions of theimpurity-removing treatment.

A transistor 460 illustrated in FIGS. 4A to 4C is an example in whichthe thickness of the oxide semiconductor film 403 is reduced by thefirst impurity-removing treatment and the second impurity-removingtreatment. FIG. 4A is a plan view of the transistor 460. FIG. 4B is across-sectional view taken along line X5-Y5 in FIG. 4A. FIG. 4C is across-sectional view taken along line X6-Y6 in FIG. 4A.

The transistor 460 is an example in which the thicknesses of regions ofthe oxide semiconductor film 403 which do not overlap with theinsulating layer 413 are reduced by the first impurity-removingtreatment, and the thicknesses of regions of the oxide semiconductorfilm 403 which do not overlap with any of the insulating layer 413, thesource electrode layer 405 a, or the drain electrode layer 405 b arereduced by the second impurity-removing treatment. Therefore, in theoxide semiconductor film 403 included in the transistor 460, regionsoverlapping with the source electrode layer 405 a and the drainelectrode layer 405 b have a larger thickness than regions which do notoverlap with any of the insulating layer 413, the source electrode layer405 a, or the drain electrode layer 405 b, and the region overlappingwith the insulating layer 413 has a larger thickness than the regionsoverlapping with the source electrode layer 405 a and the drainelectrode layer 405 b.

Note that this embodiment is not limited thereto, and for example, thethickness of part of the oxide semiconductor film 403 (the regions whichdo not overlap with the insulating layer 413) is reduced by the firstimpurity-removing treatment, and the thickness of the oxidesemiconductor film 403 is not reduced by the second impurity-removingtreatment in some cases.

In addition, by the impurity-removing treatment performed after theformation of the source electrode layer 405 a and the drain electrodelayer 405 b (or by the first impurity-removing treatment performed afterthe formation of the insulating layer 413 and the secondimpurity-removing treatment performed after the formation of the sourceelectrode layer 405 a and the drain electrode layer 405 b), the regionsof the oxide semiconductor film 403 which do not overlap with any of theinsulating layer 413, the source electrode layer 405 a, or the drainelectrode layer 405 b are removed in some cases.

A transistor 470 illustrated in FIGS. 5A to 5C is an example in whichregions of the oxide semiconductor film 403 which do not overlap withany of the insulating layer 413, the source electrode layer 405 a, orthe drain electrode layer 405 b are removed. FIG. 5A is a plan view ofthe transistor 470. FIG. 5B is a cross-sectional view taken along lineX7-Y7 in FIG. 5A. FIG. 5C is a cross-sectional view taken along lineX8-Y8 in FIG. 5A.

The transistor 470 is an example in which the regions of the oxidesemiconductor film 403 which do not overlap with any of the insulatinglayer 413, the source electrode layer 405 a, or the drain electrodelayer 405 b are removed by impurity-removing treatment. Therefore, anyregion of the oxide semiconductor film 403 included in the transistor470 overlaps with at least any one of the insulating layer 413, thesource electrode layer 405 a, and the drain electrode layer 405 b.

When the semiconductor device is manufactured using the oxidesemiconductor film 403 subjected to the impurity-removing treatment, theconcentration of the elements (e.g., chlorine, fluorine, and boron)contained in the etching gas containing the halogen element in thesurface of the oxide semiconductor film 403 (the periphery of aninterface between the oxide semiconductor film 403 and each of theinsulating layer 413, the source electrode layer 405 a, and the drainelectrode layer 405 b) can be less than or equal to 5×10¹⁸ atoms/cm³(preferably less than or equal to 1×10¹⁸ atoms/cm³).

Accordingly, a highly reliable semiconductor device including thetransistor which is formed using the oxide semiconductor film 403 andhas stable electric characteristics can be provided. In addition, thehighly reliable semiconductor device can be manufactured with a highyield, which leads to high productivity.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device and oneembodiment of a method for manufacturing the semiconductor device whichare different from those in Embodiment 1 will be described using FIGS.6A to 6C, FIGS. 7A to 7E, and FIGS. 8A to 8C. In this embodiment, atransistor including an oxide semiconductor film will be described as anexample of the semiconductor device.

The transistor may have a single-gate structure in which one channelformation region is formed, a double-gate structure in which two channelformation regions are formed, or a triple-gate structure in which threechannel formation regions are formed. Alternatively, the transistor mayhave a dual-gate structure including two gate electrode layerspositioned above and below a channel formation region with a gateinsulating film provided therebetween.

A transistor 480 illustrated in FIGS. 6A to 6C is an example of atransistor which is one of bottom-gate transistors and is also referredto as an inverted staggered transistor. FIG. 6A is a plan view of thetransistor 480. FIG. 6B is a cross-sectional view taken along line X9-Y9in FIG. 6A. FIG. 6C is a cross-sectional view taken along line X10-Y10in FIG. 6A.

The transistor 480 illustrated in FIGS. 6A to 6C includes a gateelectrode layer 401 which is provided over a substrate 400 having aninsulating surface, a gate insulating film 402 which is provided overthe gate electrode layer 401, an oxide semiconductor film 403 which hasan island shape and is provided over the gate insulating film 402, aninsulating layer 413 which is provided over the oxide semiconductor film403 and overlaps with the gate electrode layer 401, and a sourceelectrode layer 405 a and a drain electrode layer 405 b which are incontact with the oxide semiconductor film 403 and the insulating layer413. In addition, the transistor 480 may further include, as itscomponents, a base insulating film 436 which is provided over thesubstrate 400, and an interlayer insulating film 408 and a planarizationinsulating film 409 which cover the transistor 480.

In the transistor 480, a length w2 of each of the source electrode layer405 a and the drain electrode layer 405 b in the channel width directionis larger than a length w1 of the oxide semiconductor film 403 in thechannel width direction, and edge portions of the oxide semiconductorfilm 403 in the channel width direction are covered with the sourceelectrode layer 405 a and the drain electrode layer 405 b. In otherwords, regions of the oxide semiconductor film 403 which do not overlapwith the insulating layer 413 are covered with the source electrodelayer 405 a and the drain electrode layer 405 b.

The transistor 480 can have the same structure as the transistor 440except that the length w2 of each of the source electrode layer 405 aand the drain electrode layer 405 b in the channel width direction islarger than the length w1 of the oxide semiconductor film 403 in thechannel width direction.

Here, in the manufacturing process of the transistor 480, the surface ofthe oxide semiconductor film 403 is subjected to impurity-removingtreatment; therefore, elements contained in an etching gas containing ahalogen element which is used for forming the insulating layer 413 canbe prevented from remaining as impurities. Accordingly, theconcentration of the halogen element and the like in the surface of theoxide semiconductor film 403 (the periphery of an interface between theoxide semiconductor film 403 and each of the source electrode layer 405a and the drain electrode layer 405 b) can be lower than or equal to5×10¹⁸ atoms/cm³ (preferably lower than or equal to 1×10¹⁸ atoms/cm³).Specifically, the chlorine concentration thereof can be lower than orequal to 5×10¹⁸ atoms/cm³ (preferably lower than or equal to 1×10¹⁸atoms/cm³). In addition, the fluorine concentration thereof can also belower than or equal to 5×10¹⁸ atoms/cm³ (preferably lower than or equalto 1×10¹⁸ atoms/cm³), and the boron concentration thereof can also belower than or equal to 5×10¹⁸ atoms/cm³ (preferably lower than or equalto 1×10¹⁸ atoms/cm³). In this manner, since impurities which can causethe oxide semiconductor film to have lower resistance are reduced, thereliability of the semiconductor device including the oxidesemiconductor film can be increased.

A method for manufacturing such a transistor 480 is described usingFIGS. 7A to 7E.

First, the base insulating film 436, the gate electrode layer 401, thegate insulating film 402, and the oxide semiconductor film 403 areformed over the substrate 400, and an insulating film 443 is depositedto cover the oxide semiconductor film 403 (see FIG. 7A). Here, materialsand formation methods and the like of the substrate 400, the baseinsulating film 436, the gate electrode layer 401, the gate insulatingfilm 402, and the oxide semiconductor film 403 may be similar to thoseof the transistor 440 described in Embodiment 1. In addition, thematerial and the deposition method of the insulating layer 413 of thetransistor 440 can be referred to for the material and the formationmethod and the like of the insulating film 443.

Next, the insulating film 443 is processed by plasma treatment using anetching gas 442 containing a halogen element, so that the insulatinglayer 413 functioning as a channel protective film is formed in aposition overlapping with the gate electrode layer 401 (see FIG. 7B).The etching treatment is performed in such a manner that a resist mask444 is formed over the insulating film 443 by a photolithography processand selective etching is performed to form the insulating layer 413, andthen the resist mask 444 is removed. As a result, the insulating layer413 is formed to overlap with the gate electrode layer 401 and be overand in contact with a channel formation region of the oxidesemiconductor film 403.

Here, a cross-sectional shape of the insulating layer 413 is a trapezoidor a triangle, and a taper angle at a bottom edge portion of thecross-sectional shape is less than or equal to 60°, preferably less thanor equal to 45°, further preferably less than or equal to 30°.

As an etching method, dry etching is preferably used, and aparallel-plate RIE method or an ICP etching method can be used, forexample. In order to etch the film into desired shapes, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As the etching gas 442 containing the halogen element, a gas containingfluorine, a gas containing chlorine, or the like can be used. Examplesof the gas containing fluorine include carbon tetrafluoride (CF₄),sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), trifluoromethane(CHF₃), octafluorocyclobutane (C₄F₈), and the like. In the case where aninsulating film containing silicon oxide or the like is used as theinsulating film 443, etching can be easily performed using such a gascontaining fluorine. Further, examples of the gas containing chlorineinclude chlorine (Cl₂), boron trichloride (BCl₃), silicon tetrachloride(SiCl₄), or carbon tetrachloride (CCl₄), and the like. In the case wherean insulating film containing aluminum oxide or the like is used as theinsulating film 443, the etching can be easily performed using such agas containing chlorine.

Note that in this embodiment, the insulating film 443 formed usingsilicon oxide is selectively etched using CF₄ so that the taper angle ofthe bottom edge portion of the insulating layer 413 is approximately30°.

In this manner, the etching gas 442 containing the halogen element isused in the etching treatment for forming the insulating layer 413illustrated in FIG. 7B. However, when the oxide semiconductor film 403is exposed to the etching gas 442 containing the halogen element, oxygenin the oxide semiconductor film 403 is extracted by the etching gas 442containing the halogen element, and thus oxygen vacancies might beformed in the surface of the oxide semiconductor film 403 (the peripheryof the interface between the oxide semiconductor film 403 and each ofthe source electrode layer 405 a and the drain electrode layer 405 b).When oxygen vacancies are generated in the oxide semiconductor film 403,the resistance of the oxide semiconductor film 403 on the back channelside might be reduced (the oxide semiconductor film 403 on the backchannel side might have n-type conductivity) and a parasitic channelmight be formed.

For example, in the case where an oxide semiconductor materialcontaining indium is used for the oxide semiconductor film 403 and anetching gas containing boron trichloride (BCl₃) is used for processingthe insulating layer 413 which is provided in contact with the oxidesemiconductor film 403, an In—O—In bond in the oxide semiconductor filmand Cl contained in the etching gas sometimes react with each other, sothat a film including an In—Cl bond and an In element from which oxygenis detached may be formed. Since the In element from which oxygen isdetached has a dangling bond, an oxygen vacancy exists in the portion ofthe oxide semiconductor film 403, from which oxygen is detached.

Further, an element (e.g., boron) that is not halogen, which may becontained in the etching gas containing a halogen element, can cause thebackchannel of the oxide semiconductor film 403 to have lower resistance(n-type conductivity).

Therefore, impurity-removing treatment is performed after the formationof the insulating layer 413 in order to prevent elements contained inthe etching gas containing the halogen element from remaining asimpurities in the surface of the insulating layer 413 and the surface ofthe oxide semiconductor film 403 (the periphery of the interface betweenthe oxide semiconductor film 403 and each of the source electrode layer405 a and the drain electrode layer 405 b) (see FIG. 7C). Here, examplesof impurities include chlorine, fluorine, and boron.

The impurity-removing treatment can be performed by plasma treatment ortreatment with a solution. As the plasma treatment, oxygen plasmatreatment or dinitrogen monoxide plasma treatment is preferably used. Inaddition, a rare gas (typically argon) may be used in the plasmatreatment. In addition, cleaning treatment with a diluted hydrofluoricacid solution is preferably used as the treatment with the solution. Forexample, in the case where a diluted hydrofluoric acid solution is used,the dilution ratio of the diluted hydrofluoric acid is approximately1:10² to 1:10⁵, preferably approximately 1:10³ to 1:10⁵. Alternatively,as the treatment with a solution, treatment with an alkaline solutionsuch as a TMAH solution may be employed. Further, the cleaning treatmentmay be performed using water instead of a solution.

In this manner, by performing the impurity-removing treatment, theconcentration of the elements (e.g., chlorine, fluorine, and boron)contained in the etching gas containing the halogen element in thesurface of the oxide semiconductor film 403 (the periphery of theinterface between the oxide semiconductor film 403 and each of thesource electrode layer 405 a and the drain electrode layer 405 b) can belower than or equal to 5×10¹⁸ atoms/cm³ (preferably lower than or equalto 1×10¹⁸ atoms/cm³). In this manner, since the impurities which cancause the oxide semiconductor film to have lower resistance can beremoved, the reliability of the semiconductor device including the oxidesemiconductor film can be increased.

Note that heat treatment may be performed after the insulating layer 413is formed. In this embodiment, the heat treatment is performed at 300°C. for 1 hour in a nitrogen atmosphere.

Next, a conductive film is deposited over the gate electrode layer 401,the gate insulating film 402, the oxide semiconductor film 403, and theinsulating layer 413, and the conductive film is selectively etched, sothat the source electrode layer 405 a and the drain electrode layer 405b (including a wiring formed using the same layer) are formed (see FIG.7D). The source electrode layer 405 a and the drain electrode layer 405b are formed using photolithography, and the resist mask is removedafter the formation. As a result, an edge portion of the drain electrodelayer 405 b is located over a top surface or a side surface of theinsulating layer 413, and an edge portion of the source electrode layer405 a is located over the top surface or a side surface of theinsulating layer 413.

Here, the source electrode layer 405 a and the drain electrode layer 405b are formed so as to cover regions of the oxide semiconductor film 403which do not overlap with the insulating layer 413. As a result, asillustrated in FIG. 6C, the length w2 of each of the source electrodelayer 405 a and the drain electrode layer 405 b in the channel widthdirection is larger than the length w1 of the oxide semiconductor film403 in the channel width direction, and the edge portions of the oxidesemiconductor film 403 in the channel width direction are covered withthe source electrode layer 405 a and the drain electrode layer 405 b.

Here, the material formation method, and the like of the sourceelectrode layer 405 a and the drain electrode layer 405 b may be similarto those of the transistor 440 described in Embodiment 1.

In this embodiment, as the conductive film, a stack of a 100-nm-thicktitanium film, a 400-nm-thick aluminum film, and a 100-nm-thick titaniumfilm, which is formed by a sputtering method, is used.

In this embodiment, the etching of the conductive film for forming thesource electrode layer 405 a and the drain electrode layer 405 b isperformed in such a manner that the stack of the titanium film, thealuminum film, and the titanium film is etched by dry etching using Cl₂and BCl₃ as an etching gas.

In this manner, the semiconductor device is exposed to the etching gascontaining the halogen element. However, in this embodiment, the etchingof the conductive film is performed in the state where the oxidesemiconductor film is covered with the insulating layer 413, the sourceelectrode layer 405 a, and the drain electrode layer 405 b; therefore,the oxide semiconductor film can be prevented from being directlyexposed to the etching gas containing the halogen element.

Through the above-described process, the transistor 480 described inthis embodiment can be manufactured (see FIG. 7E).

In addition, as illustrated in FIG. 7E, the interlayer insulating film408 and the planarization insulating film 409 may be formed over thetransistor 480. Here, the materials and formation methods of theinterlayer insulating film 408 and the planarization insulating film 409may be similar to those of the transistor 440 described in Embodiment 1.

Note that although the transistor 480 is described as an example inwhich the thickness of the whole oxide semiconductor film 403 issubstantially uniform, this embodiment is not limited thereto. Atransistor 490 which has a different structure from the transistor 480is described using FIGS. 8A to 8C.

The transistor 490 illustrated in FIGS. 8A to 8C is an example of atransistor which is one of bottom-gate transistors and is also referredto as an inverted staggered transistor. FIG. 8A is a plan view of thetransistor 490. FIG. 8B is a cross-sectional view taken along lineX11-Y11 in FIG. 8A. FIG. 8C is a cross-sectional view taken along lineX12-Y12 in FIG. 8A.

The transistor 490 illustrated in FIGS. 8A to 8C is an example in whichregions of the oxide semiconductor film 403 which are exposed from theinsulating layer 413 are etched by the impurity-removing treatment, sothat the thicknesses of the regions are reduced.

The transistor 490 is different from the transistor 480 in that a regionof the oxide semiconductor film 403 which overlaps and is in contactwith the insulating layer 413 has a larger thickness than regions of theoxide semiconductor film 403 which overlap and are in contact with thesource electrode layer 405 a and the drain electrode layer 405 b. Notethat the other components of the transistor 490 are similar to those ofthe transistor 480; therefore, details of the components of thetransistor 480 are referred to for those of the transistor 490.

The transistor 490 can be manufactured in a manner similar to that ofthe transistor 480, and as illustrated in FIG. 7C, the impurity-removingtreatment is performed to prevent elements contained in the etching gascontaining halogen from remaining as impurities in the surface of theinsulating layer 413 and the surface of the oxide semiconductor film 403(the periphery of the interface between the oxide semiconductor film 403and each of the source electrode layer 405 a or the drain electrodelayer 405 b). The impurity-removing treatment can be performed by plasmatreatment or treatment with a solution, in a manner similar to theprocess illustrated in FIG. 7C. As the plasma treatment, oxygen plasmatreatment or dinitrogen monoxide plasma treatment is preferably used. Inaddition, a rare gas (typically argon) may be used in the plasmatreatment. In addition, cleaning treatment with a diluted hydrofluoricacid solution is preferably used as the treatment with the solution. Forexample, in the case where a diluted hydrofluoric acid solution is used,the dilution ratio of the hydrofluoric acid is approximately 1:10² to1:10⁵, preferably approximately 1:10³ to 1:10⁵. Alternatively, as thetreatment with a solution, treatment with an alkaline solution such as aTMAH solution may be employed. Further, the cleaning treatment may beperformed using water instead of a solution.

In this manner, as illustrated in FIG. 7C, plasma treatment or treatmentwith a solution may be performed on the surface of the oxidesemiconductor film 403 which is exposed to the etching gas containingthe halogen element, as the impurity-removing treatment of the surfaceof the oxide semiconductor film 403. In other words, the elements whichare contained in the etching gas containing the halogen element andremain on the surface of the oxide semiconductor film 403 as impuritiesmay be removed together with the part of the oxide semiconductor film403. As a result, the thickness of the region of the oxide semiconductorfilm 403 which overlaps with the insulating layer 413 becomes largerthan that of the region of the oxide semiconductor film 403 whichoverlaps with the source electrode layer 405 a or the drain electrodelayer 405 b. For example, when the IGZO film is processed usinghydrofluoric acid which is diluted at a ratio of 1:10³ (0.05%hydrofluoric acid), the thickness is reduced by 1 nm to 3 nm per second;when the IGZO film is processed using hydrofluoric acid which is dilutedat a ratio of 2:10⁵ (0.0025% diluted hydrofluoric acid), the thicknessis reduced by approximately 0.1 nm per second.

As described in this embodiment, when a semiconductor device includingan oxide semiconductor film is manufactured employing impurity-removingtreatment, the concentration of elements (e.g., chlorine, fluorine, andboron) contained in an etching gas containing a halogen element in asurface of the oxide semiconductor film (the periphery of an interfacebetween the oxide semiconductor film and each of a source electrodelayer and a drain electrode layer) can be lower than or equal to 5×10¹⁸atoms/cm³ (preferably lower than or equal to 1×10¹⁸ atoms/cm³).

Accordingly, a highly reliable semiconductor device including thetransistor which is formed using the oxide semiconductor film and hasstable electric characteristics can be provided. Further, a highlyreliable semiconductor device is manufactured with a high yield, whichleads to high productivity.

The methods and structures described in this embodiment can be combinedas appropriate with any of the methods and structures described in theother embodiments.

Embodiment 3

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using the transistor described inEmbodiment 1 or 2. Some or all of driver circuits including transistorscan be formed over a substrate where a pixel portion is formed, wherebya system-on-panel can be obtained.

In FIG. 9A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a first substrate 4001, and the pixel portion 4002 issealed by using a second substrate 4006. In FIG. 9A, a signal linedriver circuit 4003 and a scan line driver circuit 4004 which are eachformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared are mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. Various signals and potential aresupplied to the signal line driver circuit 4003 and the scan line drivercircuit 4004 each of which is separately formed, or the pixel portion4002 from flexible printed circuits (FPCs) 4018 a and 4018 b.

In FIGS. 9B and 9C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with the display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 9B and 9C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 9B and 9C, various signals andpotentials are supplied to the signal line driver circuit 4003 which isseparately formed, and the scan line driver circuit 4004 or the pixelportion 4002 from flexible printed circuits (FPCs) 4018.

Although FIGS. 9B and 9C each illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, one embodiment of the present invention is not limitedto this structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method or the like can beused. FIG. 9A illustrates an example in which the signal line drivercircuit 4003 and the scan line driver circuit 4004 are mounted by a COGmethod. FIG. 9B illustrates an example in which the signal line drivercircuit 4003 is mounted by a COG method. FIG. 9C illustrates an examplein which the signal line driver circuit 4003 is mounted by a TAB method.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that a display device in this specification means an image displaydevice, a display unit, or a light source (including a lighting device).Further, the display device also includes the following modules in itscategory: a module to which a connector such as an FPC, a TAB tape, or aTCP is attached; a module having a TAB tape or a TCP at the tip of whicha printed wiring board is provided; and a module in which an integratedcircuit (IC) is directly mounted on a display element by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors, and the transistordescribed in Embodiment 1 or 2 can be applied thereto.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes an elementwhose luminance is controlled by current or voltage in its category, andspecifically includes an inorganic electroluminescent (EL) element, anorganic EL element, and the like. Furthermore, a display medium whosecontrast is changed by an electric effect, such as electronic ink, canbe used.

Embodiments of the semiconductor device are described with reference toFIGS. 9A to 9C, FIGS. 10A and 10B, and FIGS. 11A and 11B. FIGS. 11A and11B are each a cross-sectional view taken along line M-N in FIG. 9B.

As shown in FIGS. 9A to 9C and FIGS. 11A and 11B, the semiconductordevice includes a connection terminal electrode 4015 and a terminalelectrode 4016, and the connection terminal electrode 4015 and theterminal electrode 4016 are electrically connected to terminals includedin the FPCs 4018 and 4018 b through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as gateelectrode layers of transistors 4010 and 4011.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of transistors. InFIGS. 11A and 11B, the transistor 4010 included in the pixel portion4002 and the transistor 4011 included in the scan line driver circuit4004 are illustrated as an example. An insulating film 4020 is providedover the transistors 4010 and 4011 in FIG. 11A, and an insulating film4021 is further provided in FIG. 11B.

The transistor described in Embodiment 1 or 2 can be applied to thetransistor 4010 and the transistor 4011. In this embodiment, an examplein which a transistor having a structure similar to that of thetransistor 440 described in Embodiment 1 is used is described. Thetransistors 4010 and 4011 are inverted staggered transistors with abottom-gate structure, in each of which an insulating layer functioningas a channel protective film is provided over an oxide semiconductorfilm.

Each of the transistors 4010 and 4011 has a structure similar to that ofthe transistor 440 described in Embodiment 1, and is obtained by amanufacturing method similar to that of the transistor 440. In themanufacturing method, a source electrode layer and a drain electrodelayer are formed through an etching step using halogen plasma, and thena step in which impurities (specifically elements contained in theetching gas) are removed from a surface of an oxide semiconductor filmand the periphery thereof is performed. In addition, theimpurity-removing treatment may be performed after an insulating filmfunctioning as a channel protective film is formed through an etchingstep using halogen plasma. For the impurity-removing treatment, forexample, diluted hydrofluoric acid treatment or plasma treatment usingoxygen or dinitrogen monoxide can be favorably employed.

Since the surface of the oxide semiconductor film and the peripherythereof can be prevented from being contaminated by impurities containedin the etching gas, the concentration of the elements (e.g., chlorineand boron) contained in the etching gas containing the halogen elementcan be lower than or equal to 5×10¹⁸ atoms/cm³ (preferably lower than orequal to 1×10¹⁸ atoms/cm³) in the surface of the oxide semiconductorfilm of each of the transistors 4010 and 4011.

Accordingly, highly reliable semiconductor devices can be provided asthe semiconductor devices of this embodiment in FIGS. 9A to 9C and FIGS.11A and 11B, which include the transistors 4010 and 4011 which areformed using an oxide semiconductor film and have stable electriccharacteristics. Further, such a highly reliable semiconductor devicecan be manufactured with a high yield, which leads to high productivity.

Further, a conductive layer may be provided in a region overlapping witha channel formation region of the oxide semiconductor film in thetransistor 4011 for the driver circuit. By providing the conductivelayer in the region overlapping with the channel formation region of theoxide semiconductor film, the amount of change in the threshold voltageof the transistor 4011 by a bias-temperature stress test (BT test) canbe further reduced. The conductive layer may have the same potential asor a potential different from that of a gate electrode layer of thetransistor 4011, and can function as a second gate electrode layer. Thepotential of the conductive layer may be GND, OV, or in a floatingstate.

The conductive layer also functions of blocking an external electricfield, that is, preventing an external electric field (particularly, toprevent static electricity) from effecting the inside (a circuit portionincluding a transistor). A blocking function of the conductive layer canprevent the variation in electrical characteristics of the transistordue to the effect of external electric field such as static electricity.

The transistor 4010 included in the pixel portion 4002 is electricallyconnected to a display element to form a display panel. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beemployed.

FIG. 11A illustrates an example of a liquid crystal display device usinga liquid crystal element as a display element. In FIG. 11A, a liquidcrystal element 4013 which is a display element includes the firstelectrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. Insulating films 4032 and 4033 serving as alignmentfilms are provided so that the liquid crystal layer 4008 is providedtherebetween. The second electrode layer 4031 is provided on the secondsubstrate 4006 side, and the first electrode layer 4030 and the secondelectrode layer 4031 are stacked, with the liquid crystal layer 4008provided therebetween.

A spacer 4035 is a columnar spacer which is obtained by selectiveetching of an insulating film and is provided in order to control thethickness (a cell gap) of the liquid crystal layer 4008. Alternatively,a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material (liquid crystalcomposition) exhibits a cholesteric phase, a smectic phase, a cubicphase, a chiral nematic phase, an isotropic phase, or the like dependingon conditions.

Alternatively, a liquid crystal composition exhibiting a blue phase forwhich an alignment film is unnecessary may be used for the liquidcrystal layer 4008. In this case, the liquid crystal layer 4008 is incontact with the first electrode layer 4030 and the second electrodelayer 4031. A blue phase is one of liquid crystal phases, which isgenerated just before a cholesteric phase changes into an isotropicphase while temperature of cholesteric liquid crystal is increased. Theblue phase can be exhibited using a liquid crystal composition which isa mixture of a liquid crystal and a chiral agent. In order to increasethe temperature range where the blue phase is exhibited, a liquidcrystal layer may be formed by adding a polymerizable monomer, apolymerization initiator, and the like to a liquid crystal compositionexhibiting a blue phase and by performing polymer stabilizationtreatment. The liquid crystal composition exhibiting a blue phase has ashort response time, and has optical isotropy, which contributes to theexclusion of the alignment process and reduction of viewing angledependence. In addition, since an alignment film does not need to beprovided and rubbing treatment is unnecessary, electrostatic dischargedamage caused by the rubbing treatment can be prevented and defects anddamage of the liquid crystal display device can be reduced in themanufacturing process. Thus, productivity of the liquid crystal displaydevice can be increased. A transistor including an oxide semiconductorfilm has a possibility that the electric characteristics of thetransistor may vary significantly by the influence of static electricityand deviate from the designed range. Therefore, it is more effective touse a liquid crystal composition exhibiting a blue phase for the liquidcrystal display device including the transistor formed using an oxidesemiconductor film.

The specific resistivity of the liquid crystal material is greater thanor equal to 1×10⁹ Ω·cm, preferably greater than or equal to 1×10¹¹ Ω·cm,further preferably greater than or equal to 1×10¹² Ω·cm. Note that thespecific resistivity in this specification is measured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. The size of the storage capacitor may be setconsidering the off-state current of a transistor or the like. By usingthe transistor including an oxide semiconductor film disclosed in thisspecification, a capacitance that is ⅓ or less, preferably ⅕ or less ofliquid crystal capacitance of each pixel is enough as the magnitude ofthe storage capacitor.

In the transistor using an oxide semiconductor film disclosed in thisspecification, the current in an off state (off-state current) can becontrolled to be low. Accordingly, an electrical signal such as an imagesignal can be held for a longer period in the pixel, and a writinginterval can be set longer in an on state. Accordingly, the frequency ofrefresh operation can be reduced, which leads to an effect ofsuppressing power consumption.

The transistor including an oxide semiconductor film disclosed in thisspecification can have relatively high field-effect mobility and thuscan operate at high speed. For example, when such a transistor which canoperate at high speed is used for a liquid crystal display device, aswitching transistor in a pixel portion and a driver transistor in adriver circuit portion can be formed over one substrate. That is, sincea semiconductor device formed of a silicon wafer or the like is notadditionally needed as a driver circuit, the number of components of thesemiconductor device can be reduced. In addition, by using a transistorwhich can operate at high speed in a pixel portion, a high-quality imagecan be provided.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. Some examples are given as the vertical alignment mode.For example, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, an Advanced Super View (ASV) mode, andthe like can be used. Furthermore, this embodiment can be applied to aVA liquid crystal display device. The VA liquid crystal display devicehas a kind of form in which alignment of liquid crystal molecules of aliquid crystal display panel is controlled. In the VA liquid crystaldisplay device, liquid crystal molecules are aligned in a verticaldirection with respect to a panel surface when no voltage is applied.Moreover, it is possible to use a method called domain multiplication ormulti-domain design, in which a pixel is divided into some regions(subpixels) and molecules are aligned in different directions in theirrespective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

As a display method in the pixel portion, a progressive method, aninterlace method or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); R, G, B, and one or more of yellow, cyan, magenta, and the like;or the like can be used. Further, the sizes of display regions may bedifferent between respective dots of color elements. Note that thedisclosed invention is not limited to the application to a displaydevice for color display; the disclosed invention can also be applied toa display device for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element. In this embodiment, an example in which anorganic EL element is used as the light-emitting element is described.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, atleast one of the pair of electrodes has a light-transmitting property.The light-emitting element can have a top emission structure in whichlight emission is extracted through the surface opposite to thesubstrate; a bottom emission structure in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure in which light emission is extracted through the surfaceopposite to the substrate and the surface on the substrate side, and alight-emitting element having any of these emission structures can beused.

Examples of a light-emitting device in which a light-emitting element isused as a display element are illustrated in FIGS. 10A and 10B and FIG.11B.

FIG. 10A is a plan view of a light-emitting device and FIG. 10B is across-sectional view taken along dashed-dotted lines V1-W1, V2-W2, andV3-W3 in FIG. 10A. Note that, an electroluminescent layer 542 and asecond electrode layer 543 are not illustrated in the plan view in FIG.10A.

The light-emitting device illustrated in FIGS. 10A and 10B includes,over a substrate 500, a transistor 510, a capacitor 520, and a wiringlayer intersection 530. The transistor 510 is electrically connected toa light-emitting element 540. Note that FIGS. 10A and 10B illustrate abottom-emission light-emitting device in which light from thelight-emitting element 540 is extracted through the substrate 500.

The transistor described in Embodiment 1 or 2 can be applied to thetransistor 510. In this embodiment, an example in which a transistorhaving a structure similar to that of the transistor 440 described inEmbodiment 1 is used is described. The transistor 510 is an invertedstaggered transistor with a bottom-gate structure, in which aninsulating layer functioning as a channel protective film is providedover an oxide semiconductor film.

The transistor 510 includes gate electrode layers 511 a and 511 b, agate insulating film 502, an oxide semiconductor film 512, an insulatinglayer 503, and conductive layers 513 a and 513 b functioning as a sourceelectrode layer and a drain electrode layer.

The transistor 510 has a structure similar to that of the transistor 440described in Embodiment 1, and is obtained by a manufacturing methodsimilar to that of the transistor 440. In the manufacturing method, theconductive layers 513 a and 513 b functioning as the source electrodelayer and the drain electrode layer are formed through an etching stepusing halogen plasma, and then a step in which impurities contained inthe etching gas are removed from a surface of the oxide semiconductorfilm and the periphery thereof is performed. The impurity-removingtreatment may be performed after an insulating layer functioning as achannel protective film is formed through an etching step using halogenplasma. For the impurity removal step, for example, diluted hydrofluoricacid treatment or plasma treatment using oxygen or dinitrogen monoxidecan be favorably employed.

Since the surface of the oxide semiconductor film and the peripherythereof can be prevented from being contaminated by impurities containedin the etching gas, the concentration of elements (e.g., chlorine andboron) contained in the etching gas containing a halogen element can belower than or equal to 5×10¹⁸ atoms/cm³ (preferably lower than or equalto 1×10¹⁸ atoms/cm³) in the surface of the oxide semiconductor film ofthe transistor 510.

Accordingly, a highly reliable semiconductor device can be provided asthe semiconductor device of this embodiment in FIGS. 10A and 10B, whichincludes the transistor 510 which is formed using the oxidesemiconductor film 512 and has stable electric characteristics. Further,such a highly reliable semiconductor device can be manufactured with ahigh yield, which leads to high productivity.

The capacitor 520 includes conductive layers 521 a and 521 b, the gateinsulating film 502, an oxide semiconductor film 522, and a conductivelayer 523. The gate insulating film 502 and the oxide semiconductor film522 are provided between the conductive layer 523 and the conductivelayers 521 a and 521 b, so that the capacitor is formed.

The wiring layer intersection 530 is an intersection of a conductivelayer 533 and the gate electrode layers 511 a and 511 b. The conductivelayer 533 and the gate electrode layers 511 a and 511 b intersect witheach other with the gate insulating film 502 and an insulating layer 553which is formed through the same step as the insulating layer 503provided therebetween. In the structure described in this embodiment,not only the gate insulating film 502 but also the insulating layer 553can be provided between the conductive layer 533 and the gate electrodelayers 511 a and 511 b at the wiring layer intersection 530; thus,parasitic capacitance generated between the conductive layer 533 and thegate electrode layers 511 a and 511 b can be reduced.

In this embodiment, a 30-nm-thick titanium film is used as the gateelectrode layer 511 a and the conductive layer 521 a, and a 200-nm-thickcopper thin film is used as the gate electrode layer 511 b and theconductive layer 521 b. Thus, the gate electrode layer has astacked-layer structure of a titanium film and a copper thin film.

A 25-nm-thick IGZO film is used as the oxide semiconductor films 512 and522.

An interlayer insulating film 504 is formed over the transistor 510, thecapacitor 520, and the wiring layer intersection 530. Over theinterlayer insulating film 504, a color filter layer 505 is provided ina region overlapping with the light-emitting element 540. An insulatingfilm 506 functioning as a planarization insulating film is provided overthe interlayer insulating film 504 and the color filter layer 505.

The light-emitting element 540 having a stacked-layer structure in whicha first electrode layer 541, the electroluminescent layer 542, and thesecond electrode layer 543 are stacked in that order is provided overthe insulating film 506. The first electrode layer 541 and theconductive layer 513 a are in contact with each other in an openingformed in the insulating film 506 and the interlayer insulating film504, which reaches the conductive layer 513 a; thus the light-emittingelement 540 and the transistor 510 are electrically connected to eachother. Note that a partition 507 is provided so as to cover part of thefirst electrode layer 541 and the opening.

As the interlayer insulating film 504, a silicon oxynitride film havinga thickness greater than or equal to 200 nm and less than or equal to600 nm, which is formed by a plasma CVD method can be used. Further, aphotosensitive acrylic film having a thickness of 1500 nm and aphotosensitive polyimide film having a thickness of 1500 nm can be usedas the insulating film 506 and the partition 507, respectively.

As the color filter layer 505, for example, a chromaticlight-transmitting resin can be used. As such a chromaticlight-transmitting resin, a photosensitive organic resin or anonphotosensitive organic resin can be used. A photosensitive organicresin layer is preferably used, because the number of resist masks canbe reduced, leading to simplification of a process.

Chromatic colors are all colors except achromatic colors such as black,gray, and white. The color filter layer is formed using a material whichtransmits only light of the chromatic colors. As chromatic color, red,green, blue, or the like can be used. Further, cyan, magenta, yellow, orthe like may also be used. “Transmitting only light of a chromaticcolor” means that light passing through the color filter layer has apeak at a wavelength of the light of the chromatic color. The thicknessof the color filter layer may be controlled as appropriate inconsideration of the relationship between the concentration of thecoloring material to be included and the transmittance of light. Forexample, the color filter layer 505 may have a thickness greater than orequal to 1500 nm and less than or equal to 2000 nm.

In the light-emitting device illustrated in FIG. 11B, a light-emittingelement 4513 which is a display element is electrically connected to thetransistor 4010 provided in the pixel portion 4002. A structure of thelight-emitting element 4513 is not limited to the illustratedstacked-layer structure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031. Thestructure of the light-emitting element 4513 can be changed asappropriate depending on a direction in which light is extracted fromthe light-emitting element 4513, or the like.

Partitions 4510 and 507 can be formed using an organic insulatingmaterial or an inorganic insulating material. It is particularlypreferable that the partitions 4510 and 507 are formed using aphotosensitive resin material to have an opening over the firstelectrode layers 4030 and 541 so that a sidewall of the opening isformed as a tilted surface with continuous curvature.

The electroluminescent layers 4511 and 542 consist of either a singlelayer or a plurality of layers stacked.

In order to prevent the entry of oxygen, hydrogen, moisture, carbondioxide, and the like into the light-emitting elements 4513 and 540, aprotective film may be formed over the second electrode layers 4031 and543 and the partitions 4510 and 507. As the protective film, a siliconnitride film, a silicon nitride oxide film, a DLC film, or the like canbe formed.

Further, a layer containing an organic compound may be deposited by adeposition method to cover the light-emitting element 4513 and 540 sothat oxygen, hydrogen, moisture, carbon dioxide, and the like do notenter the light-emitting elements 4513 and 540.

In addition, in a space which is formed with the first substrate 4001,the second substrate 4006, and the sealant 4005, a filler 4514 isprovided for sealing. It is preferable that a panel be packaged (sealed)with a protective film (such as a laminate film or an ultravioletcurable resin film) or a cover material with high air-tightness andlittle degasification so that the panel is not exposed to the outsideair, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used, as well as an inert gas such as nitrogen or argon.PVC (polyvinyl chloride), acrylic, polyimide, an epoxy resin, a siliconeresin, PVB (polyvinyl butyral), or EVA (ethylene vinyl acetate) can beused. For example, nitrogen is used for the filler.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also referred toas an electrophoretic display device (an electrophoretic display) and isadvantageous in that it has the same level of readability as plainpaper, it has lower power consumption than other display devices, and itcan be made thin and lightweight.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles and the second particles eachcontain pigment and do not move without an electric field. Moreover, thefirst particles and the second particles have different colors (whichmay be colorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese materials.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

Note that in FIGS. 9A to 9C to FIGS. 11A and 11B, a flexible substrateas well as a glass substrate can be used as any of the first substrate4001, the substrate 500, and the second substrate 4006. As plastic, afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used. In thecase where a light-transmitting property is not needed, a metalsubstrate (metal film) of aluminum, stainless steel, or the like may beused. For example, a sheet with a structure in which an aluminum foil isprovided between PVF films or polyester films can be used.

In this embodiment, an aluminum oxide film is used as the insulatingfilm 4020. The insulating film 4020 can be formed by a sputtering methodor a plasma CVD method.

The aluminum oxide film provided as the insulating film 4020 over theoxide semiconductor film has a high shielding effect (blocking effect)of preventing penetration of both oxygen and impurities such as hydrogenand moisture.

Therefore, during the manufacturing process and after the manufacture,the aluminum oxide film functions as a protective film for preventingthe entry of an impurity such as hydrogen or moisture, which can cause achange, into the oxide semiconductor film and release of oxygen, whichis a main component material of the oxide semiconductor, from the oxidesemiconductor film.

The insulating films 4021 and 506 each serving as a planarizationinsulating film can be formed using an organic material having heatresistance, such as an acrylic resin, polyimide, benzocyclobutene-basedresin, polyamide, or epoxy. Other than such organic materials, it isalso possible to use a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. The insulating film may beformed by stacking a plurality of insulating films formed of thesematerials.

The method for forming the insulating films 4021 and 506 is notparticularly limited, and the following method can be used depending onthe material: a sputtering method, an SOG method, a spin coating method,a dipping method, a spray coating method, a droplet discharge method(such as an ink-jet method), a printing method (such as screen printingor offset printing), or the like. Further, the insulating films 4021 and506 can be formed with a doctor knife, a roll coater, a curtain coater,a knife coater, or the like.

The display device displays an image by transmitting light from a lightsource or a display element. Therefore, the substrate and the thin filmssuch as the insulating film and the conductive film provided for thepixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible-light wavelength range.

The first electrode layer and the second electrode layer (each of whichmay be called a pixel electrode layer, a common electrode layer, acounter electrode layer, or the like) for applying voltage to thedisplay element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,the pattern structure of the electrode layer, and the like.

The first electrode layers 4030 and 541 and the second electrode layers4031 and 543 can be formed using a light-transmitting conductivematerial such as indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide (referredto as ITO), indium zinc oxide, indium tin oxide to which silicon oxideis added, or graphene.

Alternatively, the first electrode layers 4030 and 541 and the secondelectrode layers 4031 and 543 can be formed using one or more materialsselected from metals such as tungsten (W), molybdenum (Mo), zirconium(Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium(Cr), cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum(Al), copper (Cu), and silver (Ag); an alloy of any of these metals; anda nitride of any of these metals.

In this embodiment, since the light-emitting device illustrated in FIGS.10A and 10B has a bottom-emission structure, the first electrode layer541 has a light-transmitting property and the second electrode layer 543has a light-reflecting property. Accordingly, in the case of using ametal film as the first electrode layer 541, the film is preferably thinenough to secure a light-transmitting property; in the case of using alight-transmitting conductive film as the second electrode layer 543, aconductive film having a light-reflecting property is preferably stackedtherewith.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the first electrodelayers 4030 and 541 and the second electrode layers 4031 and 543. As theconductive high molecule, what is called π-electron conjugatedconductive polymer can be used. Examples thereof include polyaniline ora derivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, a copolymer of two or more kinds of aniline,pyrrole, and thiophene or a derivative thereof.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

By using any of the transistors described in Embodiment 1 or 2 asdescribed above, the semiconductor device can have a variety offunctions.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 4

A semiconductor device having an image sensor function of readinginformation on an object can be manufactured using the transistordescribed in Embodiment 1 or 2.

FIG. 12A illustrates an example of a semiconductor device having animage sensor function. FIG. 12A illustrates an equivalent circuit of aphoto sensor, and FIG. 12B is a cross-sectional view illustrating partof the photo sensor.

In a photodiode 602, one electrode is electrically connected to aphotodiode reset signal line 658, and the other electrode iselectrically connected to a gate of a transistor 640. One of a sourceand a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and thedrain thereof is electrically connected to one of a source and a drainof a transistor 656. A gate of the transistor 656 is electricallyconnected to a gate signal line 659, and the other of the source and thedrain thereof is electrically connected to a photo sensor output signalline 671.

Note that in circuit diagrams in this specification, a transistorincluding an oxide semiconductor film is denoted by a symbol “OS” sothat it can be identified as a transistor including an oxidesemiconductor film. In FIG. 12A, the transistor 640 and the transistor656 are each a transistor including an oxide semiconductor film, towhich any of the transistor described in Embodiments 1 or 2 can beapplied. In this embodiment, an example in which a transistor having astructure similar to that of the transistor 440 described in Embodiment1 is used is described. The transistors 640 is an inverted staggeredtransistor with a bottom-gate structure, in which an insulating layerfunctioning as a channel protective film is provided over an oxidesemiconductor film.

FIG. 12B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photo sensor. The photodiode 602 functioning as asensor and the transistor 640 are provided over a substrate 601 (a TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 with an adhesive layer608 provided therebetween.

An insulating film 631, an interlayer insulating film 633, and aninterlayer insulating film 634 are provided over the transistor 640. Thephotodiode 602 is provided over the interlayer insulating film 633. Inthe photodiode 602, a first semiconductor film 606 a, a secondsemiconductor film 606 b, and a third semiconductor film 606 c arestacked in this order from the interlayer insulating film 633 side,between electrode layers 641 a and 641 b provided over the interlayerinsulating film 633 and an electrode layer 642 provided over theinterlayer insulating film 634.

The electrode layer 641 b is electrically connected to a conductivelayer 643 formed in the interlayer insulating film 634, and theelectrode layer 642 is electrically connected to a conductive layer 645through an electrode layer 641 a. The conductive layer 645 iselectrically connected to a gate electrode layer of the transistor 640,and the photodiode 602 is electrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor film having p-typeconductivity as the first semiconductor film 606 a, a high-resistancesemiconductor film (i-type semiconductor film) as the secondsemiconductor film 606 b, and a semiconductor film having n-typeconductivity as the third semiconductor film 606 c are stacked isillustrated as an example.

The first semiconductor film 606 a is a p-type semiconductor film andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity. The first semiconductor film 606a is formed by a plasma CVD method with the use of a semiconductorsource gas containing an impurity element belonging to Group 13 (e.g.,boron (B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with use of a diffusionmethod or an ion injecting method. Heating or the like may be conductedafter introducing the impurity element by an ion injecting method or thelike in order to diffuse the impurity element. In this case, as a methodof forming the amorphous silicon film, an LPCVD method, a chemical vapordeposition method, a sputtering method, or the like may be used. Thefirst semiconductor film 606 a is preferably formed to have a thicknessgreater than or equal to 10 nm and less than or equal to 50 nm.

The second semiconductor film 606 b is an i-type semiconductor film(intrinsic semiconductor film) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor film 606 b, anamorphous silicon film is formed by a plasma CVD method with the use ofa semiconductor source gas. As the semiconductor source gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor film 606 b may beformed by an LPCVD method, a vapor deposition method, a sputteringmethod, or the like. The second semiconductor film 606 b is preferablyformed to have a thickness greater than or equal to 200 nm and less thanor equal to 1000 nm.

The third semiconductor film 606 c is an n-type semiconductor film andis formed using an amorphous silicon film containing an impurity elementimparting n-type conductivity type. The third semiconductor film 606 cis formed by a plasma CVD method with the use of a semiconductor sourcegas containing an impurity element belonging to Group 15 (e.g.,phosphorus (P)). As the semiconductor material gas, silane (SiH₄) may beused. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the likemay be used. Further alternatively, an amorphous silicon film which doesnot contain an impurity element may be formed, and then, an impurityelement may be introduced to the amorphous silicon film with use of adiffusion method or an ion injecting method. Heating or the like may beconducted after introducing the impurity element by an ion injectingmethod or the like in order to diffuse the impurity element. In thiscase, as a method of forming the amorphous silicon film, an LPCVDmethod, a chemical vapor deposition method, a sputtering method, or thelike may be used. The third semiconductor film 606 c is preferablyformed to have a thickness greater than or equal to 20 nm and less thanor equal to 200 nm.

The first semiconductor film 606 a, the second semiconductor film 606 b,and the third semiconductor film 606 c are not necessarily formed usingan amorphous semiconductor, and may be formed using a polycrystallinesemiconductor or a microcrystalline semiconductor (semi-amorphoussemiconductor: SAS).

In addition, the mobility of holes generated by the photoelectric effectis lower than the mobility of electrons. Therefore, a pin photodiode hasbetter characteristics when a surface on the p-type semiconductor filmside is used as a light-receiving plane. Here, an example in which lightreceived by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalsis described. Further, light from the semiconductor film having aconductivity type opposite to that of the semiconductor film on thelight-receiving plane is disturbance light; therefore, the electrodelayer is preferably formed using a light-blocking conductive film. Notethat the n-type semiconductor film side may alternatively be alight-receiving plane.

The insulating film 631, the interlayer insulating film 633, and theinterlayer insulating film 634 can be formed using an insulatingmaterial by a sputtering method, a plasma CVD method, an SOG method,spin coating, dipping, spray coating, a droplet discharge method (suchas an inkjet method), a printing method (such as screen printing oroffset printing), or the like depending on the material.

The insulating film 631 can be formed using an inorganic insulatingmaterial and can have a single-layer structure or a stacked structureincluding any of oxide insulating films such as a silicon oxide layer, asilicon oxynitride layer, an aluminum oxide layer, and an aluminumoxynitride layer; and nitride insulating films such as a silicon nitridelayer, a silicon nitride oxide layer, an aluminum nitride layer, and analuminum nitride oxide layer.

In this embodiment, an aluminum oxide film is used as the insulatingfilm 631. The insulating film 631 can be formed by a sputtering methodor a plasma CVD method.

The aluminum oxide film provided as the insulating film 631 over theoxide semiconductor film has a high shielding effect (blocking effect)and thus is less likely to transmit both oxygen and an impurity such ashydrogen or moisture.

Therefore, during the manufacturing process and after the manufacture,the aluminum oxide film functions as a protective film for preventingthe entry of an impurity such as hydrogen or moisture, which can cause achange, into the oxide semiconductor film and release of oxygen, whichis a main component material of the oxide semiconductor, from the oxidesemiconductor film.

For a reduction in surface roughness, an insulating film functioning asa planarization insulating film is preferably used as each of theinterlayer insulating films 633 and 634. For the interlayer insulatingfilms 633 and 634, for example, an organic insulating material havingheat resistance such as polyimide, an acrylic resin, a benzocyclobuteneresin, polyamide, or an epoxy resin can be used. Other than such organicinsulating materials, it is possible to use a single layer or stackedlayers of a low-dielectric constant material (a low-k material), asiloxane-based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like.

With detection of light 622 that enters the photodiode 602, data on anobject to be detected can be read. Note that a light source such as abacklight can be used at the time of reading information on an object.

The transistor 640 has a structure similar to that of the transistor 440described in Embodiment 1, and is obtained by a manufacturing methodsimilar to that of the transistor 440. In the manufacturing method, asource electrode layer and a drain electrode layer are formed through anetching step using halogen plasma, and then a step in which impuritiescontained in the etching gas are removed from a surface of the oxidesemiconductor film and the periphery thereof is performed. Theimpurity-removing treatment may be performed after an insulating layerfunctioning as a channel protective film is formed through an etchingstep using halogen plasma. For the impurity removal step, for example,dilute hydrofluoric acid treatment or plasma treatment using oxygen ordinitrogen monoxide can be favorably employed.

Since the surface of the oxide semiconductor film and the peripherythereof can be prevented from being contaminated by impurities containedin the etching gas, the halogen element concentration can be lower thanor equal to 5×10¹⁸ atoms/cm³ (preferably lower than or equal to 1×10¹⁸atoms/cm³) in the surface of the oxide semiconductor film of thetransistor 640.

Accordingly, a highly reliable semiconductor device including thetransistor 640 of this embodiment, which is formed using an oxidesemiconductor film and has stable electric characteristics, can beprovided. Further, a highly reliable semiconductor device can bemanufactured with a high yield, which leads to high productivity.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 5

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof the electronic appliances include a television set (also referred toas a television or a television receiver), a monitor of a computer,cameras such as a digital camera and a digital video camera, a digitalphoto frame, a mobile phone, a portable game machine, a personal digitalassistant, an audio reproducing device, a game machine (e.g., a pachinkomachine or a slot machine), a game console, and the like. Specificexamples of these electronic appliances are illustrated in FIGS. 13A to13C.

FIG. 13A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

The semiconductor device described in any of Embodiments 1 to 4 can beused for the display portion 9003 so that the electronic appliances canhave a high reliability.

The display portion 9003 has a touch-input function. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her fingers or the like, the user can carryout operation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the display portion 9003 may function as a controldevice which controls the home appliances by operation on the screen.For example, with the use of the semiconductor device having an imagesensor function described in Embodiment 4, the display portion 9003 canhave a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. Atelevision set with a large screen takes up too much space that isavailable in a small room. However, with a table having a displayportion therein, it is possible to make the use of the space in theroom.

FIG. 13B illustrates a television set 9100. In the television set 9100,a display portion 9103 is incorporated in a housing 9101 and an imagecan be displayed on the display portion 9103. Note that the housing 9101is supported by a stand 9105 here.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Furthermore, the remote controller 9110 may be provided witha display portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 13B is provided with areceiver, a modem, and the like. With the use of the receiver, thetelevision set 9100 can receive general TV broadcasts. Moreover, whenthe television set 9100 is connected to a communication network with orwithout wires via the modem, one-way (from a sender to a receiver) ortwo-way (between a sender and a receiver or between receivers)information communication can be performed.

The semiconductor device described in any of Embodiments 1 to 4 can beused for the display portions 9103 and 9107 so that the television setand the remote controller can have high reliability.

FIG. 13C illustrates a computer which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of Embodiments 1 to 4 can beused for the display portion 9203 so that the computer can have highreliability.

FIGS. 14A and 14B illustrate a tablet terminal that can be folded. InFIG. 14A, the tablet terminal is opened, and includes a housing 9630, adisplay portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of Embodiments 1 to 4 can beused for the display portion 9631 a and the display portion 9631 b sothat the tablet terminal can have high reliability.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9638 that aredisplayed. Note that FIG. 14A shows, as an example, that half of thearea of the display portion 9631 a has only a display function and theother half of the area has a touch panel function. However, thestructure of the display portion 9631 a is not limited to this, and allthe area of the display portion 9631 a may have a touch panel function.For example, all the area of the display portion 9631 a can displaykeyboard buttons and serve as a touch panel while the display portion9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 can switch display orientation(e.g., between landscape mode and portrait mode) and select a displaymode (switch between monochrome display and color display), for example.With the power-saving-mode switching button 9036, the luminance ofdisplay can be optimized in accordance with the amount of external lightat the time when the tablet is in use, which is detected with an opticalsensor incorporated in the tablet. The tablet may include anotherdetection device such as a sensor for detecting orientation (e.g., agyroscope or an acceleration sensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 14A, an embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

The tablet terminal is closed in FIG. 14B. The tablet terminal includesthe housing 9630, a solar battery 9633, a charge/discharge controlcircuit 9634, a battery 9635, and a DCDC converter 9636. Note that inFIG. 14B, a structure including a battery 9635 and a DCDC converter 9636is illustrated as an example of the charge/discharge control circuit9634.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, thereby providing a tabletterminal with high endurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 14A and 14B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

Power can be supplied to the touch panel, the display portion, an imagesignal processor, and the like by the solar battery 9633 attached on asurface of the tablet terminal Note that the solar battery 9633 can beprovided on one or two surfaces of the housing 9630, so that the battery9635 can be charged efficiently. When a lithium ion battery is used asthe battery 9635, there is an advantage of downsizing or the like.

The structure and operation of the charge/discharge control circuit 9634illustrated in FIG. 14B are described with reference to a block diagramof FIG. 14C. FIG. 14C illustrates the solar battery 9633, the battery9635, the DCDC converter 9636, a converter 9637, switches SW1 to SW3,and the display portion 9631. The battery 9635, the DCDC converter 9636,the converter 9637, and the switches SW1 to SW3 correspond to thecharge/discharge control circuit 9634 in FIG. 14B.

First, an example of operation in the case where power is generated bythe solar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by theDCDC converter 9636 so that a voltage for charging the battery 9635 isobtained. When the display portion 9631 is operated with the power fromthe solar battery 9633, the switch SW1 is turned on and the voltage ofthe power is raised or lowered by the converter 9637 to a voltage neededfor operating the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off anda switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Example 1

In this example, a metal film was formed over and in contact with anoxide semiconductor film, and then dry etching was performed to removethe metal film. The following experiment was conducted to examine therelation between resistivity and whether or not an impurity-removingtreatment is performed to remove an impurity generated by the dryetching.

First, as a comparative sample, a 95-nm-thick IGZO film was formed overa glass substrate with the use of a deposition apparatus using asputtering method, and the resistivity of the IGZO film was measured.The result was 4.8×10⁹ Ω·cm. The resistivity of the IGZO film wasobtained in the following manner: an electrode (a stack of a100-nm-thick titanium film, a 400-nm-thick aluminum film, and a100-nm-thick titanium film) having a meandering top shape was formed,and the resistance was obtained by a voltage-current two-wiremeasurement.

The IGZO film was formed under the following condition: an oxide targetof In:Ga:Zn=1:1:1 [atomic ratio] was used, the atmosphere was anatmosphere of oxygen and argon (oxygen flow rate: 50%), the pressure was0.6 Pa, the power of the AC power source was 5 kW, and the substratetemperature was 170° C.

A deposition apparatus using a sputtering method includes a sputteringchamber in which the pressure can be reduced by a vacuum evacuation unitsuch as a vacuum pump (e.g., a cryopump or a turbo molecular pump), asubstrate holder on which a substrate to be processed is fixed, a targetholder which holds a sputtering target, an electrode for the sputteringtarget held by the target holder, a power supply unit which applies ACvoltage (or DC voltage) for sputtering to the electrode, and a gassupply unit which supplies a gas to the sputtering chamber. Infabrication of the sample, the sputtering chamber is kept under highvacuum so as to prevent entry of impurities as much as possible, andfilm formation is performed in a dry nitrogen atmosphere in which, interms of moisture, the dew point is −40° C. or lower, preferably −50° C.or lower.

Sample 1 was obtained in such a manner that a 95-nm-thick IGZO film wasformed over a glass substrate, etching was performed under a first dryetching condition for 180 seconds, and the substrate with the film wassoaked in pure water. Then an electrode was formed and the resistivitywas measured. The result of Sample 1 was 130 Ω·cm. Sample 2 was obtainedby soaking the substrate with film in diluted hydrofluoric acid (dilutedat a ratio of 1:100) for 30 seconds after the etching under the firstdry etching condition. Then an electrode was formed and the resistivitywas measured. The result of Sample 2 was 3.9×10⁹ Ω·cm.

These results confirm the following: dry etching using a gas containinga halogen element causes an impurity to be attached to or mixed into theIGZO film, so that the resistivity thereof is lowered; however, theimpurity is removed by surface treatment using dilute hydrofluoric acid,so that the IGZO film has a state close to that before the dry etching.

Sample 3 was obtained in such a manner that a 95-nm-thick IGZO film wasformed over a glass substrate, etching was performed under a second dryetching condition for 180 seconds, and the substrate with the film wassoaked in pure water. Then an electrode was formed and the resistivitywas measured. Sample 4 was obtained by soaking the substrate with thefilm in diluted hydrofluoric acid (diluted at a ratio of 1:100) for 30seconds after the etching under the second dry etching condition. Thenan electrode was formed and the resistivity was measured.

Sample 5 was obtained in such a manner that a 95-nm-thick IGZO film wasformed over a glass substrate, etching was performed under a third dryetching condition for 180 seconds, and the substrate with the film wassoaked in pure water. Then, an electrode was formed and the resistivitywas measured. Sample 6 was obtained by soaking the substrate with thefilm in diluted hydrofluoric acid (diluted at a ratio of 1:100) for 30seconds after the etching under the third dry etching condition. Then anelectrode was formed and the resistivity was measured.

Sample 7 was obtained in such a manner that a 95-nm-thick IGZO film wasformed over a glass substrate, etching was performed under a fourth dryetching condition for 180 seconds, and the substrate with the film wassoaked in pure water. Then, an electrode was formed and the resistivitywas measured. Sample 8 was obtained by soaking the substrate with thefilm in diluted hydrofluoric acid (diluted at a ratio of 1:100) for 30seconds after the etching under the fourth dry etching condition. Thenan electrode was formed and the resistivity was measured.

Table 1 shows the first dry etching condition, the second dry etchingcondition, the third dry etching condition, and the fourth dry etchingcondition. As an apparatus for dry etching, an ICP etching apparatus wasused.

TABLE 1 ICP Bias Pressure Cl₂ BCl₃ SF₆ O₂ Time (W) (W) (Pa) (sccm)(sccm) (sccm) (sccm) (sec) First etching 2000  200 2.0 — — 900 100 180condition Second etching 2000 1000 2.5 540 — 540 — condition Thirdetching   0 1500 2.0 150 750 — — condition Fourth etching 2000 1000 2.5— 380 700 — condition

In a graph of FIG. 16, the vertical axis represents resistivity and theresistivity of the comparative sample and the resistivities of Samples 1to 8 are shown. The results confirm that, regardless of dry etchingcondition, surface treatment using dilute hydrofluoric acid enables theIGZO film to have a state close to, preferably the same as, that beforethe dry etching.

This application is based on Japanese Patent Application serial no.2011-233171 filed with Japan Patent Office on Oct. 24, 2011 and JapanesePatent Application serial no. 2011-233274 filed with Japan Patent Officeon Oct. 24, 2011, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A method for manufacturing a semiconductordevice, comprising the steps of: forming a gate electrode layer over aninsulating surface; forming a gate insulating film over the gateelectrode layer; forming an oxide semiconductor film over the gateinsulating film; forming an insulating layer over the oxidesemiconductor film, the insulating layer overlapping with the gateelectrode layer; forming a conductive film over the oxide semiconductorfilm and the insulating layer; etching the conductive film with anetching gas containing a halogen element to form a source electrodelayer and a drain electrode layer; and removing the halogen element fromthe oxide semiconductor film by oxygen plasma treatment or dinitrogenmonoxide plasma treatment.
 2. The method for manufacturing asemiconductor device, according to claim 1, wherein a chlorineconcentration in the oxide semiconductor film subjected to the removingstep is lower than or equal to 5×10¹⁸ atoms/cm³.
 3. A method formanufacturing a semiconductor device, comprising the steps of: forming agate electrode layer over an insulating surface; forming a gateinsulating film over the gate electrode layer; forming an oxidesemiconductor film over the gate insulating film; forming an insulatinglayer over the oxide semiconductor film; etching the insulating layerwith an etching gas containing a halogen element to form a channelprotective film in a position overlapping with the gate electrode layer;removing the halogen element from the oxide semiconductor film by oxygenplasma treatment or dinitrogen monoxide plasma treatment; forming aconductive film over the oxide semiconductor film and the channelprotective film; and etching the conductive film to form a sourceelectrode layer and a drain electrode layer.
 4. The method formanufacturing a semiconductor device, according to claim 3, wherein achlorine concentration in the oxide semiconductor film subjected to theremoving step is lower than or equal to 5×10¹⁸ atoms/cm³.
 5. A methodfor manufacturing a semiconductor device, comprising the steps of:forming a gate electrode layer over an insulating surface; forming agate insulating film over the gate electrode layer; forming an oxidesemiconductor film over the gate insulating film; forming an insulatinglayer over the oxide semiconductor film; etching the insulating layerwith a first etching gas containing a halogen element to form a channelprotective film in a position overlapping with the gate electrode layer;removing the halogen element contained in the first etching gas from theoxide semiconductor film; forming a conductive film over the oxidesemiconductor film and the channel protective film; etching theconductive film with a second etching gas containing a halogen elementto form a source electrode layer and a drain electrode layer; andremoving the halogen element contained in the second etching gas fromthe oxide semiconductor film.
 6. The method for manufacturing asemiconductor device, according to claim 5, wherein a chlorineconcentration in the oxide semiconductor film subjected to the removingstep with the second etching gas is lower than or equal to 5×10¹⁸atoms/cm³.
 7. The method for manufacturing a semiconductor device,according to claim 5, wherein oxygen plasma treatment or dinitrogenmonoxide plasma treatment is performed as at least one of the removingstep with the first etching gas and the removing step with the secondetching gas.
 8. The method for manufacturing a semiconductor device,according to claim 5, wherein cleaning treatment with a dilutedhydrofluoric acid solution is performed as at least one of the removingstep with the first etching gas and the removing step with the secondetching gas.